diff --git a/CHANGELOG.md b/CHANGELOG.md
index ff2b679..3e83018 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,55 @@
 # Changelog
 
+## v4.0.0 — 2024-09
+
+### Potentially breaking change: Default decoded GeoJSON to SRID 4326 (WGS 84)
+
+This aligns our GeoJSON decoding with [the GeoJSON spec](https://tools.ietf.org/html/rfc7946#section-4) by making all decoded GeoJSON infer the WGS 84 datum (SRID 4326) by default. Whereas previously when you called `Geo.JSON.decode/1` or `decode!/1`, we would return geometries with an `:srid` of `nil`, we now return `srid: 4326`. Likewise when encoding GeoJSON, we explicitly output a `crs` field indicating the datum.
+
+This is unlikely to break real-world usage unless your implementation was assuming a different datum by default.
+
+A couple examples of the changes:
+
+**Before**:
+
+```elixir
+iex> Geo.JSON.decode!(%{"type" => "Point", "coordinates" => [1.0, 2.0]})
+%Geo.Point{
+  coordinates: {1.0, 2.0},
+  # Note the default SRID
+  srid: nil
+}
+```
+
+**After**
+
+```elixir
+iex> Geo.JSON.decode!(%{"type" => "Point", "coordinates" => [1.0, 2.0]})
+%Geo.Point{
+  coordinates: {1.0, 2.0},
+  # New explicit default
+  srid: 4326
+}
+```
+
+If you were then encode this value again, you'd end up with a new `crs` field in the output GeoJSON:
+
+```elixir
+iex> %{"type" => "Point", "coordinates" => [1.0, 2.0]}
+...> |> Geo.JSON.decode!()
+...> |> GeoJSON.encode!()
+%{
+  "type" => "Point",
+  "coordinates" => [1.0, 2.0],
+  # Note the new `crs` field which was not present in the input to Geo.JSON.decode!/1
+  "crs" => %{"properties" => %{"name" => "EPSG:4326"}, "type" => "name"}
+}
+```
+
+This last behavior is the most potentially troublesome. However, we don't have a good way of distinguishing a case where you explicitly had the `crs` set in the input to the decoding function (in which case you would probably also like to have it present in the re-encoded version) compared to one in which it's been inferred.
+
+Thanks to @gworkman for reporting this issue ([#129](https://github.com/felt/geo/issues/129)).
+
 ## v3.6.0 — 2023-10-19
 
 As of v3.6.0, `geo` (like [`geo_postgis`](https://github.com/felt/geo_postgis)) is being maintained by the Felt team. As a company building a geospatial product on Elixir, with a track record of [supporting open source software](https://felt.com/open-source), we're excited for the future of the project.
diff --git a/README.md b/README.md
index 1febc84..438c823 100644
--- a/README.md
+++ b/README.md
@@ -33,7 +33,7 @@ _Note_: If you are looking to do geospatial calculations in memory with Geo's st
 ```elixir
 defp deps do
   [
-    {:geo, "~> 3.6"}
+    {:geo, "~> 4.0"}
   ]
 end
 ```
diff --git a/lib/geo/json.ex b/lib/geo/json.ex
index 58352f3..89c9d4d 100644
--- a/lib/geo/json.ex
+++ b/lib/geo/json.ex
@@ -7,13 +7,16 @@ defmodule Geo.JSON do
   so that you can use the resulting GeoJSON structure as a property
   in larger JSON structures.
 
+  Note that, per [the GeoJSON spec](https://tools.ietf.org/html/rfc7946#section-4),
+  all geometries are assumed to use the WGS 84 datum (SRID 4326) by default.
+
   ## Examples
 
       # Using Jason as the JSON parser for these examples
 
       iex> json = "{ \\"type\\": \\"Point\\", \\"coordinates\\": [100.0, 0.0] }"
       ...> json |> Jason.decode!() |> Geo.JSON.decode!()
-      %Geo.Point{coordinates: {100.0, 0.0}, srid: nil}
+      %Geo.Point{coordinates: {100.0, 0.0}, srid: 4326}
 
       iex> geom = %Geo.Point{coordinates: {100.0, 0.0}, srid: nil}
       ...> Jason.encode!(geom)
diff --git a/lib/geo/json/decoder.ex b/lib/geo/json/decoder.ex
index 673bc6a..8deba5e 100644
--- a/lib/geo/json/decoder.ex
+++ b/lib/geo/json/decoder.ex
@@ -96,6 +96,9 @@ defmodule Geo.JSON.Decoder do
       true ->
         raise DecodeError, value: geo_json
     end
+    # Per #129, the GeoJSON spec says all GeoJSON coordinates default to SRID 4326 (WGS 84)
+    # https://tools.ietf.org/html/rfc7946#section-4
+    |> default_srid_4326()
   end
 
   @doc """
@@ -279,4 +282,12 @@ defmodule Geo.JSON.Decoder do
   defp ensure_numeric(other) do
     raise ArgumentError, "expected a numeric coordinate, got: #{inspect(other)}"
   end
+
+  defp default_srid_4326(%{srid: nil} = geom), do: %{geom | srid: 4326}
+
+  defp default_srid_4326(%{geometries: geometries} = geom) when is_list(geometries) do
+    %{geom | geometries: Enum.map(geometries, &default_srid_4326/1)}
+  end
+
+  defp default_srid_4326(geom), do: geom
 end
diff --git a/mix.exs b/mix.exs
index 16be361..2a5146c 100644
--- a/mix.exs
+++ b/mix.exs
@@ -2,7 +2,7 @@ defmodule Geo.Mixfile do
   use Mix.Project
 
   @source_url "https://github.com/felt/geo"
-  @version "3.6.0"
+  @version "4.0.0"
 
   def project do
     [
diff --git a/test/geo/json_test.exs b/test/geo/json_test.exs
index 193be51..90726ff 100644
--- a/test/geo/json_test.exs
+++ b/test/geo/json_test.exs
@@ -36,7 +36,7 @@ defmodule Geo.JSON.Test do
     json = "{\"type\":\"Point\",\"coordinates\":[100.0,0.0,70.0]}"
     geom = Jason.decode!(json) |> Geo.JSON.decode!()
 
-    assert(geom == %Geo.PointZ{coordinates: {100.0, 0.0, 70.0}})
+    assert geom == %Geo.PointZ{coordinates: {100.0, 0.0, 70.0}, srid: 4326}
   end
 
   test "LineString to GeoJson" do
@@ -54,7 +54,7 @@ defmodule Geo.JSON.Test do
     assert(geom.coordinates == {100.0, 0.0})
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson Point without coordinates" do
@@ -64,7 +64,7 @@ defmodule Geo.JSON.Test do
     assert(is_nil(geom.coordinates))
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson with SRID to Point and back" do
@@ -78,7 +78,7 @@ defmodule Geo.JSON.Test do
     assert(geom.srid == 4326)
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to LineString and back" do
@@ -88,7 +88,7 @@ defmodule Geo.JSON.Test do
 
     assert(geom.coordinates == [{100.0, 0.0}, {101.0, 1.0}])
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to LineStringZ and back" do
@@ -100,7 +100,7 @@ defmodule Geo.JSON.Test do
 
     assert(geom.coordinates == [{100.0, 0.0, 50.0}, {101.0, 1.0, 20.0}])
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "Drops M coordinate when decoding LineStringZM" do
@@ -145,7 +145,7 @@ defmodule Geo.JSON.Test do
     )
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to MultiPoint and back" do
@@ -155,7 +155,7 @@ defmodule Geo.JSON.Test do
 
     assert(geom.coordinates == [{100.0, 0.0}, {101.0, 1.0}])
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to MultiLineString and back" do
@@ -167,7 +167,7 @@ defmodule Geo.JSON.Test do
 
     assert(geom.coordinates == [[{100.0, 0.0}, {101.0, 1.0}], [{102.0, 2.0}, {103.0, 3.0}]])
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to MultiLineStringZ and back" do
@@ -185,7 +185,7 @@ defmodule Geo.JSON.Test do
     )
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to MultiPolygon and back" do
@@ -206,7 +206,7 @@ defmodule Geo.JSON.Test do
     )
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJson to GeometryCollection and back" do
@@ -219,7 +219,7 @@ defmodule Geo.JSON.Test do
     assert(Enum.count(geom.geometries) == 2)
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "Unable to encode non-geo type" do
@@ -281,7 +281,7 @@ defmodule Geo.JSON.Test do
     assert(Enum.count(geom.geometries) == 2)
 
     new_exjson = Geo.JSON.encode!(geom)
-    assert(exjson == new_exjson)
+    assert_geojson_equal(exjson, new_exjson)
   end
 
   test "GeoJSON to GeometryCollection" do
@@ -454,7 +454,16 @@ defmodule Geo.JSON.Test do
             y <- float()
           ) do
       geom = %Geo.Point{coordinates: {x, y}}
-      assert geom == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
+      assert %{geom | srid: 4326} == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
+
+      geom_with_srid_and_props = %Geo.Point{
+        coordinates: {x, y},
+        srid: 1234,
+        properties: %{"foo" => "bar"}
+      }
+
+      assert %{geom_with_srid_and_props | srid: 1234} ==
+               Geo.JSON.encode!(geom_with_srid_and_props) |> Geo.JSON.decode!()
     end
   end
 
@@ -463,13 +472,13 @@ defmodule Geo.JSON.Test do
     json = Geo.JSON.encode!(geom) |> Jason.encode!()
 
     assert(json == "{\"coordinates\":[],\"type\":\"Point\"}")
-    assert geom == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
+    assert %{geom | srid: 4326} == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
   end
 
   property "encodes and decodes back to the correct LineString struct" do
     check all(list <- list_of({float(), float()}, min_length: 1)) do
       geom = %Geo.LineString{coordinates: list}
-      assert geom == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
+      assert %{geom | srid: 4326} == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
     end
   end
 
@@ -588,4 +597,21 @@ defmodule Geo.JSON.Test do
     assert Enum.all?(geom.geometries, &match?(%Geo.MultiPolygon{}, &1))
     assert geom.properties["id"] == "FLC017"
   end
+
+  defp assert_geojson_equal(%{} = json_1, %{} = json_2) do
+    # Per the GeoJSON spec, GeoJSON is assumed to have WGS 84 datum (SRID 4326) by default
+    assert drop_srid_4326(json_1) == drop_srid_4326(json_2),
+           "Inequivalent GeoJSON values:\n" <>
+             "Left:\n" <>
+             "#{inspect(json_1, pretty: true)}\n" <>
+             "Right:\n" <>
+             "#{inspect(json_2, pretty: true)}"
+  end
+
+  defp drop_srid_4326(%{"crs" => crs} = json)
+       when crs == %{"properties" => %{"name" => "EPSG:4326"}, "type" => "name"} do
+    Map.delete(json, "crs")
+  end
+
+  defp drop_srid_4326(%{} = json), do: json
 end