diff --git a/src/libANGLE/renderer/metal/mtl_command_buffer.h b/src/libANGLE/renderer/metal/mtl_command_buffer.h
--- a/src/libANGLE/renderer/metal/mtl_command_buffer.h
+++ b/src/libANGLE/renderer/metal/mtl_command_buffer.h
@@ -560,7 +560,7 @@ class RenderCommandEncoder final : public CommandEncoder
                                                                      const BufferRef &indexBuffer,
                                                                      size_t bufferOffset,
                                                                      uint32_t instances,
-                                                                     uint32_t baseVertex,
+                                                                     int32_t baseVertex,
                                                                      uint32_t baseInstance);

     RenderCommandEncoder &setVisibilityResultMode(MTLVisibilityResultMode mode, size_t offset);
diff --git a/src/libANGLE/renderer/metal/mtl_command_buffer.mm b/src/libANGLE/renderer/metal/mtl_command_buffer.mm
--- a/src/libANGLE/renderer/metal/mtl_command_buffer.mm
+++ b/src/libANGLE/renderer/metal/mtl_command_buffer.mm
@@ -358,7 +358,9 @@ inline void DrawIndexedInstancedBaseVertexBaseInstanceCmd(id<MTLRenderCommandEnc
     id<MTLBuffer> indexBuffer      = stream->fetch<id<MTLBuffer>>();
     size_t bufferOffset            = stream->fetch<size_t>();
     uint32_t instances             = stream->fetch<uint32_t>();
-    uint32_t baseVertex            = stream->fetch<uint32_t>();
+    // baseVertex is signed at the GL API (GLint) and at the Metal API
+    // (NSInteger). Preserve sign so negative values sign-extend to int64.
+    int32_t baseVertex             = stream->fetch<int32_t>();
     uint32_t baseInstance          = stream->fetch<uint32_t>();
     [encoder drawIndexedPrimitives:primitiveType
                         indexCount:indexCount
@@ -2199,7 +2201,7 @@ RenderCommandEncoder &RenderCommandEncoder::drawIndexedInstancedBaseVertexBaseIn
     const BufferRef &indexBuffer,
     size_t bufferOffset,
     uint32_t instances,
-    uint32_t baseVertex,
+    int32_t baseVertex,
     uint32_t baseInstance)
 {
     ASSERT(mPipelineStateSet &&