diff --git a/.github/workflows/add_label_from_diff.yaml b/.github/workflows/add_label_from_diff.yaml
new file mode 100644
index 0000000000000..271ac1b95ce59
--- /dev/null
+++ b/.github/workflows/add_label_from_diff.yaml
@@ -0,0 +1,42 @@
+name: Autolabel PRs
+
+on:
+  pull_request:
+    types: [opened]
+  push:
+    paths:
+      - scripts/autolabel.lean
+      - .github/workflows/add_label_from_diff.yaml
+
+jobs:
+  add_topic_label:
+    name: Add topic label
+    runs-on: ubuntu-latest
+    # Don't run on forks, where we wouldn't have permissions to add the label anyway.
+    if: github.repository == 'leanprover-community/mathlib4'
+    permissions:
+      issues: write
+      checks: write
+      pull-requests: write
+      contents: read
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - name: install elan
+        run: |
+          set -o pipefail
+          curl -sSfL https://github.com/leanprover/elan/releases/download/v3.1.1/elan-x86_64-unknown-linux-gnu.tar.gz | tar xz
+          ./elan-init -y --default-toolchain none
+          echo "$HOME/.elan/bin" >> "${GITHUB_PATH}"
+      - name: lake exe autolabel
+        run: |
+          # the checkout dance, to avoid a detached head
+          git checkout master
+          git checkout -
+          lake exe autolabel "$NUMBER"
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GH_REPO: ${{ github.repository }}
+          NUMBER: ${{ github.event.number }}
diff --git a/Mathlib.lean b/Mathlib.lean
index 3442f8704e094..ddf844378e225 100644
--- a/Mathlib.lean
+++ b/Mathlib.lean
@@ -3475,6 +3475,7 @@ import Mathlib.NumberTheory.Zsqrtd.Basic
 import Mathlib.NumberTheory.Zsqrtd.GaussianInt
 import Mathlib.NumberTheory.Zsqrtd.QuadraticReciprocity
 import Mathlib.NumberTheory.Zsqrtd.ToReal
+import Mathlib.Numerology.Everything
 import Mathlib.Order.Antichain
 import Mathlib.Order.Antisymmetrization
 import Mathlib.Order.Atoms
diff --git a/Mathlib/Numerology/Everything.lean b/Mathlib/Numerology/Everything.lean
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/lakefile.lean b/lakefile.lean
index 0b406c4741783..1af1f1e8b764a 100644
--- a/lakefile.lean
+++ b/lakefile.lean
@@ -80,6 +80,16 @@ lean_lib docs where
 ## Executables provided by Mathlib
 -/
 
+/--
+`lake exe autolabel 150100` adds a topic label to PR `150100` if there is a unique choice.
+This requires GitHub CLI `gh` to be installed!
+
+Calling `lake exe autolabel` without a PR number will print the result without applying
+any labels online.
+-/
+lean_exe autolabel where
+  srcDir := "scripts"
+
 /-- `lake exe cache get` retrieves precompiled `.olean` files from a central server. -/
 lean_exe cache where
   root := `Cache.Main
diff --git a/scripts/autolabel.lean b/scripts/autolabel.lean
new file mode 100644
index 0000000000000..ccf7d7321c35e
--- /dev/null
+++ b/scripts/autolabel.lean
@@ -0,0 +1,310 @@
+/-
+Copyright (c) 2024 Damiano Testa. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Jon Eugster, Damiano Testa
+-/
+import Lean.Elab.Command
+
+/-!
+# Automatic labelling of PRs
+
+This file contains the script to automatically assign a GitHub label to a PR.
+
+## Label definition
+
+The mapping from GitHub labels to Mathlib folders is done in this file and
+needs to be updated here if necessary:
+
+* `AutoLabel.mathlibLabels` contains an assignment of GitHub labels to folders inside
+  the mathlib repository. If no folder is specified, a label like `t-set-theory` will be
+  interpreted as matching the folder `"Mathlib" / "SetTheory"`.
+* `AutoLabel.mathlibUnlabelled` contains subfolders of `Mathlib/` which are deliberately
+  left without topic label.
+
+## lake exe autolabel
+
+`lake exe autolabel` uses `git diff --name-only origin/master...HEAD` to determine which
+files have been modifed and then finds all labels which should be added based on these changes.
+These are printed for testing purposes.
+
+`lake exe autolabel [NUMBER]` will further try to add the applicable labels
+to the PR specified. This requires the **GitHub CLI** `gh` to be installed!
+Example: `lake exe autolabel 10402` for PR #10402.
+
+For the time being, the script only adds a label if it finds a **single unique label**
+which would apply. If multiple labels are found, nothing happens.
+
+## Workflow
+
+There is a mathlib workflow `.github/workflows/add_label_from_diff.yaml` which executes
+this script automatically.
+
+Currently it is set to run only one time when a PR is created.
+
+## Tests
+
+Additionally, the script does a few consistency checks:
+
+- it ensures all paths in specified in `AutoLabel.mathlibLabels` exist
+- It makes sure all subfolders of `Mathlib/` belong to at least one label.
+  There is `AutoLabel.mathlibUnlabelled` to add exceptions for this test.
+
+-/
+
+open Lean System
+
+namespace AutoLabel
+
+/--
+A `Label` consists of the
+* The `label` field is the actual GitHub label name.
+* The `dirs` field is the array of all "root paths" such that a modification in a file contained
+  in one of these paths should be labelled with `label`.
+* The `exclusions` field is the array of all "root paths" that are excluded, among the
+  ones that start with the ones in `dirs`.
+  Any modifications to a file in an excluded path is ignored for the purposes of labelling.
+-/
+structure Label where
+  /-- The label name as it appears on GitHub -/
+  label : String
+  /-- Array of paths which fall under this label. e.g. `"Mathlib" / "Algebra"`.
+
+  For a label of the form `t-set-theory` this defaults to `#["Mathlib" / "SetTheory"]`. -/
+  dirs : Array FilePath := if label.startsWith "t-" then
+      #["Mathlib" / ("".intercalate (label.splitOn "-" |>.drop 1 |>.map .capitalize))]
+    else #[]
+  /-- Array of paths which should be excluded.
+  Any modifications to a file in an excluded path are ignored for the purposes of labelling. -/
+  exclusions : Array FilePath := #[]
+  deriving BEq, Hashable
+
+/--
+Mathlib labels and their corresponding folders. Add new labels and folders here!
+-/
+def mathlibLabels : Array Label := #[
+  { label := "t-algebra",
+    dirs := #[
+      "Mathlib" / "Algebra",
+      "Mathlib" / "FieldTheory",
+      "Mathlib" / "RingTheory",
+      "Mathlib" / "GroupTheory",
+      "Mathlib" / "RepresentationTheory",
+      "Mathlib" / "LinearAlgebra"] },
+  { label := "t-algebraic-geometry",
+    dirs := #[
+      "Mathlib" / "AlgebraicGeometry",
+      "Mathlib" / "Geometry" / "RingedSpace"] },
+  { label := "t-analysis" },
+  { label := "t-category-theory" },
+  { label := "t-combinatorics" },
+  { label := "t-computability" },
+  { label := "t-condensed" },
+  { label := "t-data" },
+  { label := "t-differential-geometry",
+    dirs := #["Mathlib" / "Geometry" / "Manifold"] },
+  { label := "t-dynamics" },
+  { label := "t-euclidean-geometry",
+    dirs := #["Mathlib" / "Geometry" / "Euclidean"] },
+  { label := "t-linter",
+    dirs := #["Mathlib" / "Tactic" / "Linter"] },
+  { label := "t-logic",
+    dirs := #[
+      "Mathlib" / "Logic",
+      "Mathlib" / "ModelTheory"] },
+  { label := "t-measure-probability",
+    dirs := #[
+      "Mathlib" / "MeasureTheory",
+      "Mathlib" / "Probability",
+      "Mathlib" / "InformationTheory"] },
+  { label := "t-meta",
+    dirs := #[
+      "Mathlib" / "Control",
+      "Mathlib" / "Lean",
+      "Mathlib" / "Mathport",
+      "Mathlib" / "Tactic",
+      "Mathlib" / "Util"],
+    exclusions := #["Mathlib" / "Tactic" / "Linter"] },
+  { label := "t-number-theory" },
+  { label := "t-order" },
+  { label := "t-set-theory" },
+  { label := "t-topology",
+    dirs := #[
+      "Mathlib" / "Topology",
+      "Mathlib" / "AlgebraicTopology"] },
+  { label := "CI",
+    dirs := #[".github"] },
+  { label := "IMO",
+    dirs := #["Archive" / "Imo"] } ]
+
+/-- Exceptions inside `Mathlib/` which are not covered by any label. -/
+def mathlibUnlabelled : Array FilePath := #[
+    "Mathlib" / "Deprecated",
+    "Mathlib" / "Init",
+    "Mathlib" / "Testing",
+    "Mathlib" / "Std" ]
+
+/-- Checks if the folder `path` lies inside the folder `dir`. -/
+def _root_.System.FilePath.isPrefixOf (dir path : FilePath) : Bool :=
+  -- use `dir / ""` to prevent partial matching of folder names
+  (dir / "").normalize.toString.isPrefixOf (path / "").normalize.toString
+
+/--
+Return all names of labels in `mathlibLabels` which match
+at least one of the `files`.
+
+* `files`: array of relative paths starting from the mathlib root directory.
+-/
+def getMatchingLabels (files : Array FilePath) : Array String :=
+  let applicable := mathlibLabels.filter fun label ↦
+    -- first exclude all files the label excludes,
+    -- then see if any file remains included by the label
+    let notExcludedFiles := files.filter fun file ↦
+      label.exclusions.all (!·.isPrefixOf file)
+    label.dirs.any (fun dir ↦ notExcludedFiles.any (dir.isPrefixOf ·))
+  -- return sorted list of label names
+  applicable.map (·.label) |>.qsort (· < ·)
+
+/-!
+Testing the functionality of the declarations defined in this script
+-/
+section Tests
+
+-- Test `FilePath.isPrefixOf`
+#guard ("Mathlib" / "Algebra" : FilePath).isPrefixOf ("Mathlib" / "Algebra" / "Basic.lean")
+
+-- Test `FilePath.isPrefixOf` does not trigger on partial prefixes
+#guard ! ("Mathlib" / "Algebra" : FilePath).isPrefixOf ("Mathlib" / "AlgebraicGeometry")
+
+#guard getMatchingLabels #[] == #[]
+-- Test default value for `label.dirs` works
+#guard getMatchingLabels #["Mathlib" / "SetTheory" / "ZFC"] == #["t-set-theory"]
+-- Test exclusion
+#guard getMatchingLabels #["Mathlib" / "Tactic"/ "Abel.lean"] == #["t-meta"]
+#guard getMatchingLabels #["Mathlib" / "Tactic"/ "Linter" / "Lint.lean"] == #["t-linter"]
+#guard getMatchingLabels #[
+  "Mathlib" / "Tactic"/ "Linter" / "Lint.lean",
+  "Mathlib" / "Tactic" / "Abel.lean" ] == #["t-linter", "t-meta"]
+
+/-- Testing function to ensure the labels defined in `mathlibLabels` cover all
+subfolders of `Mathlib/`. -/
+partial def findUncoveredPaths (path : FilePath) (exceptions : Array FilePath := #[]) :
+    IO <| Array FilePath := do
+  let mut notMatched : Array FilePath := #[]
+  -- all directories inside `path`
+  let subDirs ← (← path.readDir).map (·.path) |>.filterM (do FilePath.isDir ·)
+  for dir in subDirs do
+    -- if the sub directory is not matched by a label,
+    -- we go recursively into it
+    if (getMatchingLabels #[dir]).size == 0 then
+      notMatched := notMatched ++ (← findUncoveredPaths dir exceptions)
+  -- a directory should be flagged if none of its sub-directories is matched by a label
+  -- note: we assume here the base directory, i.e. "Mathlib" is never matched by a label,
+  -- therefore we skip this test.
+  if notMatched.size == subDirs.size then
+    if exceptions.contains path then
+      return #[]
+    else
+      return #[path]
+  else
+    return notMatched
+
+end Tests
+
+/--
+Create a message which GitHub CI parses as annotation and displays at the specified file.
+
+Note: `file` is duplicated below so that it is also visible in the plain text output.
+
+* `type`: "error" or "warning"
+* `file`: file where the annotation should be displayed
+* `title`: title of the annotation
+* `message`: annotation message
+-/
+def githubAnnotation (type file title message : String) : String :=
+  s!"::{type} file={file},title={title}::{file}: {message}"
+
+end AutoLabel
+
+open IO AutoLabel in
+
+/-- `args` is expected to have length 0 or 1, where the first argument is the PR number.
+
+If a PR number is provided, the script requires GitHub CLI `gh` to be installed in order
+to add the label to the PR.
+
+## Exit codes:
+
+- `0`: success
+- `1`: invalid arguments provided
+- `2`: invalid labels defined
+- `3`: ~labels do not cover all of `Mathlib/`~ (unused; only emitting warning)
+-/
+unsafe def main (args : List String): IO Unit := do
+  if args.length > 1 then
+    println s!"::error:: autolabel: invalid number of arguments ({args.length}), \
+    expected at most 1. Please run without arguments or provide the target PR's \
+    number as a single argument!"
+    IO.Process.exit 1
+  let prNumber? := args[0]?
+
+  -- test: validate that all paths in `mathlibLabels` actually exist
+  let mut valid := true
+  for label in mathlibLabels do
+    for dir in label.dirs do
+      unless ← FilePath.pathExists dir do
+        -- print github annotation error
+        println <| AutoLabel.githubAnnotation "error" "scripts/autolabel.lean"
+          s!"Misformatted `{ ``AutoLabel.mathlibLabels }`"
+          s!"directory '{dir}' does not exist but is included by label '{label.label}'. \
+          Please update `{ ``AutoLabel.mathlibLabels }`!"
+        valid := false
+    for dir in label.exclusions do
+      unless ← FilePath.pathExists dir do
+        -- print github annotation error
+        println <| AutoLabel.githubAnnotation "error" "scripts/autolabel.lean"
+          s!"Misformatted `{ ``AutoLabel.mathlibLabels }`"
+          s!"directory '{dir}' does not exist but is excluded by label '{label.label}'. \
+          Please update `{ ``AutoLabel.mathlibLabels }`!"
+        valid := false
+  unless valid do
+    IO.Process.exit 2
+
+  -- test: validate that the labels cover all of the `Mathlib/` folder
+  let notMatchedPaths ← findUncoveredPaths "Mathlib" (exceptions := mathlibUnlabelled)
+  if notMatchedPaths.size > 0 then
+    -- print github annotation warning
+    -- note: only emitting a warning because the workflow is only triggered on the first commit
+    -- of a PR and could therefore lead to unexpected behaviour if a folder was created later.
+    println <| AutoLabel.githubAnnotation "warning" "scripts/autolabel.lean"
+      s!"Incomplete `{ ``AutoLabel.mathlibLabels }`"
+      s!"the following paths inside `Mathlib/` are not covered \
+      by any label: {notMatchedPaths} Please modify `AutoLabel.mathlibLabels` accordingly!"
+    -- IO.Process.exit 3
+
+  -- get the modified files
+  let gitDiff ← IO.Process.run {
+    cmd := "git",
+    args := #["diff", "--name-only", "origin/master...HEAD"] }
+  let modifiedFiles : Array FilePath := (gitDiff.splitOn "\n").toArray.map (⟨·⟩)
+
+  -- find labels covering the modified files
+  let labels := getMatchingLabels modifiedFiles
+
+  match labels with
+  | #[] =>
+    println s!"No applicable labels found!"
+  | #[label] =>
+    println s!"Exactly one label found: {label}"
+    match prNumber? with
+    | some n =>
+      let _ ← IO.Process.run {
+        cmd := "gh",
+        args := #["pr", "edit", n, "--add-label", label] }
+      println s!"Added label: {label}"
+    | none =>
+      println s!"No PR-number provided, skipping adding labels. \
+      (call `lake exe autolabel 150602` to add the labels to PR `150602`)"
+  | labels =>
+    println s!"Multiple labels found: {labels}"
+    println s!"Not adding any label."
+  IO.Process.exit 0