diff --git a/image.go b/image.go
index 30cd54fce42c..ad099dd5cbaa 100644
--- a/image.go
+++ b/image.go
@@ -535,6 +535,8 @@ type DrawTrianglesShaderOptions struct {
 	// If the uniform variable type is an array, a vector or a matrix,
 	// you have to specify linearly flattened values as a slice or an array.
 	// For example, if the uniform variable type is [4]vec4, the length will be 16.
+	//
+	// If a uniform variable's name doesn't exist in Uniforms, this is treated as if zero values are specified.
 	Uniforms map[string]any
 
 	// Images is a set of the source images.
@@ -710,6 +712,8 @@ type DrawRectShaderOptions struct {
 	// If the uniform variable type is an array, a vector or a matrix,
 	// you have to specify linearly flattened values as a slice or an array.
 	// For example, if the uniform variable type is [4]vec4, the length will be 16.
+	//
+	// If a uniform variable's name doesn't exist in Uniforms, this is treated as if zero values are specified.
 	Uniforms map[string]any
 
 	// Images is a set of the source images.
diff --git a/internal/ui/shader.go b/internal/ui/shader.go
index d5d7cb473409..ddd2909d4d93 100644
--- a/internal/ui/shader.go
+++ b/internal/ui/shader.go
@@ -55,6 +55,9 @@ func (s *Shader) AppendUniforms(dst []uint32, uniforms map[string]any) []uint32
 	origLen := len(dst)
 	if cap(dst)-len(dst) >= s.uniformUint32Count {
 		dst = dst[:len(dst)+s.uniformUint32Count]
+		for i := origLen; i < len(dst); i++ {
+			dst[i] = 0
+		}
 	} else {
 		dst = append(dst, make([]uint32, s.uniformUint32Count)...)
 	}
diff --git a/shader_test.go b/shader_test.go
index 7aa9fe30e4c1..b553cb3fdb56 100644
--- a/shader_test.go
+++ b/shader_test.go
@@ -1960,3 +1960,55 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 		})
 	}
 }
+
+// Issue #2709
+func TestShaderUniformDefaultValue(t *testing.T) {
+	const w, h = 16, 16
+
+	dst := ebiten.NewImage(w, h)
+	s, err := ebiten.NewShader([]byte(`//kage:unit pixel
+
+package main
+
+var U vec4
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	return U
+}
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Draw with a uniform variable value.
+	op := &ebiten.DrawRectShaderOptions{}
+	op.Uniforms = map[string]any{
+		"U": [...]float32{1, 1, 1, 1},
+	}
+	dst.DrawRectShader(w, h, s, op)
+
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			got := dst.At(i, j).(color.RGBA)
+			want := color.RGBA{0xff, 0xff, 0xff, 0xff}
+			if got != want {
+				t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+
+	// Draw without a uniform variable value. In this case, the uniform variable value should be 0.
+	dst.Clear()
+	op.Uniforms = nil
+	dst.DrawRectShader(w, h, s, op)
+
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			got := dst.At(i, j).(color.RGBA)
+			want := color.RGBA{0, 0, 0, 0}
+			if got != want {
+				t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+}