diff --git a/THIRD-PARTY-NOTICES.md b/THIRD-PARTY-NOTICES.md
index 8339c5f..54e280f 100644
--- a/THIRD-PARTY-NOTICES.md
+++ b/THIRD-PARTY-NOTICES.md
@@ -1,5 +1,15 @@
 # Third-Party Notices
 
+## 2D Simplex Noise
+
+`math.simplex_noise` and `cultmath_simplex_noise` port the textureless 2D
+simplex-noise function by Ian McEwan and Ashima Arts, as distributed by Stefan
+Gustavson's `webgl-noise` project and Unity Mathematics.
+
+- Copyright 2011 Ashima Arts.
+- Upstream: <https://github.com/ashima/webgl-noise>
+- License: MIT.
+
 ## Advanced Terrain Erosion Filter
 
 `src/CultMath/AdvancedErosionFilter.cs` and
diff --git a/shaders/CultMath.hlsl b/shaders/CultMath.hlsl
index cf76c8c..84d244b 100644
--- a/shaders/CultMath.hlsl
+++ b/shaders/CultMath.hlsl
@@ -73,6 +73,42 @@ float cultmath_smootherstep(float value)
     return value * value * value * (value * (value * 6.0 - 15.0) + 10.0);
 }
 
+float2 cultmath_simplex_mod289(float2 value) { return value - floor(value * (1.0 / 289.0)) * 289.0; }
+float3 cultmath_simplex_mod289(float3 value) { return value - floor(value * (1.0 / 289.0)) * 289.0; }
+float3 cultmath_simplex_permute(float3 value) { return cultmath_simplex_mod289(((value * 34.0) + 1.0) * value); }
+
+float cultmath_simplex_noise(float2 value)
+{
+    const float4 c = float4(
+        0.211324865405187,
+        0.366025403784439,
+        -0.577350269189626,
+        0.024390243902439);
+    float2 i = floor(value + dot(value, c.yy));
+    float2 x0 = value - i + dot(i, c.xx);
+    float2 i1 = x0.x > x0.y ? float2(1.0, 0.0) : float2(0.0, 1.0);
+    float4 x12 = float4(x0.x + c.x - i1.x, x0.y + c.x - i1.y, x0.x + c.z, x0.y + c.z);
+
+    i = cultmath_simplex_mod289(i);
+    float3 p = cultmath_simplex_permute(
+        cultmath_simplex_permute(i.y + float3(0.0, i1.y, 1.0)) + i.x + float3(0.0, i1.x, 1.0));
+    float3 m = max(0.5 - float3(dot(x0, x0), dot(x12.xy, x12.xy), dot(x12.zw, x12.zw)), 0.0);
+    m *= m;
+    m *= m;
+
+    float3 x = 2.0 * frac(p * c.w) - 1.0;
+    float3 h = abs(x) - 0.5;
+    float3 ox = floor(x + 0.5);
+    float3 a0 = x - ox;
+    m *= 1.79284291400159 - 0.85373472095314 * (a0 * a0 + h * h);
+
+    float3 g = float3(
+        a0.x * x0.x + h.x * x0.y,
+        a0.y * x12.x + h.y * x12.y,
+        a0.z * x12.z + h.z * x12.w);
+    return 130.0 * dot(m, g);
+}
+
 float cultmath_lengthsq(float2 value) { return dot(value, value); }
 float cultmath_lengthsq(float3 value) { return dot(value, value); }
 float cultmath_lengthsq(float4 value) { return dot(value, value); }
diff --git a/src/CultMath/math.cs b/src/CultMath/math.cs
index 874554a..e15fbf0 100644
--- a/src/CultMath/math.cs
+++ b/src/CultMath/math.cs
@@ -327,6 +327,50 @@ public static class math
     public static float hash(float2 value) => hash(dot(value, new float2(127.1f, 311.7f)));
     public static float hash(float3 value) => hash(dot(value, new float3(127.1f, 311.7f, 74.7f)));
 
+    // Ashima Arts / Ian McEwan 2D simplex noise, kept component-explicit so
+    // the C# and HLSL mirrors preserve the same float32 evaluation order.
+    public static float simplex_noise(float2 value)
+    {
+        var c = new float4(
+            0.211324865405187f,
+            0.366025403784439f,
+            -0.577350269189626f,
+            0.024390243902439f);
+        var i = floor(value + dot(value, new float2(c.y, c.y)));
+        var x0 = value - i + dot(i, new float2(c.x, c.x));
+        var i1 = x0.x > x0.y ? new float2(1.0f, 0.0f) : new float2(0.0f, 1.0f);
+        var x12 = new float4(
+            x0.x + c.x - i1.x,
+            x0.y + c.x - i1.y,
+            x0.x + c.z,
+            x0.y + c.z);
+
+        i = simplex_mod289(i);
+        var p = simplex_permute(simplex_permute(i.y + new float3(0.0f, i1.y, 1.0f)) + i.x + new float3(0.0f, i1.x, 1.0f));
+        var m = max(0.5f - new float3(
+            dot(x0, x0),
+            x12.x * x12.x + x12.y * x12.y,
+            x12.z * x12.z + x12.w * x12.w), 0.0f);
+        m = m * m;
+        m = m * m;
+
+        var x = 2.0f * frac(p * c.w) - 1.0f;
+        var h = abs(x) - 0.5f;
+        var ox = floor(x + 0.5f);
+        var a0 = x - ox;
+        m *= 1.79284291400159f - 0.85373472095314f * (a0 * a0 + h * h);
+
+        var g = new float3(
+            a0.x * x0.x + h.x * x0.y,
+            a0.y * x12.x + h.y * x12.y,
+            a0.z * x12.z + h.z * x12.w);
+        return 130.0f * dot(m, g);
+    }
+
+    private static float2 simplex_mod289(float2 value) => value - floor(value * (1.0f / 289.0f)) * 289.0f;
+    private static float3 simplex_mod289(float3 value) => value - floor(value * (1.0f / 289.0f)) * 289.0f;
+    private static float3 simplex_permute(float3 value) => simplex_mod289(((value * 34.0f) + 1.0f) * value);
+
     public static float value_noise(float2 position)
     {
         var cell = floor(position);
diff --git a/tests/CultMath.Tests/HlslMirrorTests.cs b/tests/CultMath.Tests/HlslMirrorTests.cs
index 5e1451c..47f5897 100644
--- a/tests/CultMath.Tests/HlslMirrorTests.cs
+++ b/tests/CultMath.Tests/HlslMirrorTests.cs
@@ -51,6 +51,7 @@ public sealed class HlslMirrorTests
             "cultmath_cubic_bezier",
             "cultmath_hash",
             "cultmath_value_noise",
+            "cultmath_simplex_noise",
             "cultmath_value_noise_bicubic",
             "cultmath_value_noise_texture",
             "cultmath_value_noise_texture_bicubic",
diff --git a/tests/CultMath.Tests/MathTests.cs b/tests/CultMath.Tests/MathTests.cs
index bb816d4..6979bdd 100644
--- a/tests/CultMath.Tests/MathTests.cs
+++ b/tests/CultMath.Tests/MathTests.cs
@@ -235,6 +235,16 @@ public sealed class MathTests
         Assert.True(bicubic > -0.25f && bicubic < 1.25f);
     }
 
+    [Theory]
+    [InlineData(0.0f, 0.0f, 0.0f)]
+    [InlineData(0.25f, -0.5f, -0.425866783f)]
+    [InlineData(12.25f, -4.5f, 0.258312f)]
+    [InlineData(536.5106f, 536.5106f, -0.6901103f)]
+    public void SimplexNoiseMatchesUnityMathematics(float x, float y, float expected)
+    {
+        Assert.Equal(expected, math.simplex_noise(new float2(x, y)), precision: 6);
+    }
+
     [Fact]
     public void VoronoiBatchProducesToneMappedColors()
     {