diff --git a/src/webgl/material.js b/src/webgl/material.js
index 3fa865b64a..894785c8c6 100644
--- a/src/webgl/material.js
+++ b/src/webgl/material.js
@@ -2590,6 +2590,11 @@ function material(p5, fn) {
    * <a href="#/p5/loadModel">`loadModel()`</a> apply their own normal map from
    * the `.mtl` file's `map_Bump`.
    *
+   * Note: On a shape whose texture coordinates wrap all the way around, such as
+   * <a href="#/p5/sphere">`sphere()`</a>, the two edges of the image meet. The
+   * image has to tile for them to line up, otherwise a seam shows where they
+   * join.
+   *
    * Note: `normalTexture()` can only be used in WebGL mode.
    *
    * @method normalTexture
@@ -2682,6 +2687,13 @@ function material(p5, fn) {
    *
    * A light source is needed to see the effect.
    *
+   * Note: On a shape whose texture coordinates wrap all the way around, such as
+   * <a href="#/p5/sphere">`sphere()`</a>, the two edges of the image meet. The
+   * image has to tile for them to line up. Because a bump map is read by
+   * comparing neighbouring pixels, also call
+   * <a href="#/p5/textureWrap">`textureWrap(REPEAT)`</a> so those comparisons
+   * carry across the join instead of stopping at the edge.
+   *
    * Note: `bumpTexture()` can only be used in WebGL mode.
    *
    * @method bumpTexture
diff --git a/src/webgl/shaders/phong.frag b/src/webgl/shaders/phong.frag
index 527826f460..64e39d0b2c 100644
--- a/src/webgl/shaders/phong.frag
+++ b/src/webgl/shaders/phong.frag
@@ -76,12 +76,17 @@ void main(void) {
     vec3 mapN;
     if (uNormalMapMode == 1) {
       // bump map: brightness is height, so the tangent-space normal comes from
-      // how fast that height changes between neighbouring texels.
-      float h = TEXTURE(uNormalSampler, vTexCoord).r;
-      float hu = TEXTURE(uNormalSampler, vTexCoord + vec2(uNormalTexelSize.x, 0.0)).r;
-      float hv = TEXTURE(uNormalSampler, vTexCoord + vec2(0.0, uNormalTexelSize.y)).r;
+      // how fast that height changes between neighbouring texels. sampling both
+      // sides keeps the slope right at the edges of the map, where reaching past
+      // one side would otherwise clamp and read back the same texel.
+      vec2 du = vec2(uNormalTexelSize.x, 0.0);
+      vec2 dv = vec2(0.0, uNormalTexelSize.y);
+      float hl = TEXTURE(uNormalSampler, vTexCoord - du).r;
+      float hr = TEXTURE(uNormalSampler, vTexCoord + du).r;
+      float hd = TEXTURE(uNormalSampler, vTexCoord - dv).r;
+      float hu = TEXTURE(uNormalSampler, vTexCoord + dv).r;
       // the surface leans away from the direction height increases in
-      mapN = normalize(vec3(h - hu, h - hv, 1.0));
+      mapN = normalize(vec3((hl - hr) * 0.5, (hd - hu) * 0.5, 1.0));
     } else {
       // normal map: rgb already holds the tangent-space normal
       mapN = TEXTURE(uNormalSampler, vTexCoord).rgb * 2.0 - 1.0;
diff --git a/src/webgpu/shaders/material.js b/src/webgpu/shaders/material.js
index 8541967642..5288c3b9cf 100644
--- a/src/webgpu/shaders/material.js
+++ b/src/webgpu/shaders/material.js
@@ -401,12 +401,17 @@ ${useTextureMaps ? `  if (material.uHasNormalMap == 1) {
     var mapN: vec3<f32>;
     if (material.uNormalMapMode == 1u) {
       // bump map: brightness is height, so the tangent-space normal comes from
-      // how fast that height changes between neighbouring texels.
-      let h = textureSample(uNormalSampler, uNormalSampler_sampler, input.vTexCoord).r;
-      let hu = textureSample(uNormalSampler, uNormalSampler_sampler, input.vTexCoord + vec2<f32>(material.uNormalTexelSize.x, 0.0)).r;
-      let hv = textureSample(uNormalSampler, uNormalSampler_sampler, input.vTexCoord + vec2<f32>(0.0, material.uNormalTexelSize.y)).r;
+      // how fast that height changes between neighbouring texels. sampling both
+      // sides keeps the slope right at the edges of the map, where reaching past
+      // one side would otherwise clamp and read back the same texel.
+      let du = vec2<f32>(material.uNormalTexelSize.x, 0.0);
+      let dv = vec2<f32>(0.0, material.uNormalTexelSize.y);
+      let hl = textureSample(uNormalSampler, uNormalSampler_sampler, input.vTexCoord - du).r;
+      let hr = textureSample(uNormalSampler, uNormalSampler_sampler, input.vTexCoord + du).r;
+      let hd = textureSample(uNormalSampler, uNormalSampler_sampler, input.vTexCoord - dv).r;
+      let hu = textureSample(uNormalSampler, uNormalSampler_sampler, input.vTexCoord + dv).r;
       // the surface leans away from the direction height increases in
-      mapN = normalize(vec3<f32>(h - hu, h - hv, 1.0));
+      mapN = normalize(vec3<f32>((hl - hr) * 0.5, (hd - hu) * 0.5, 1.0));
     } else {
       // normal map: rgb already holds the tangent-space normal
       mapN = textureSample(uNormalSampler, uNormalSampler_sampler, input.vTexCoord).rgb * 2.0 - 1.0;