diff --git a/src/compiler/translator/ParseContext.cpp b/src/compiler/translator/ParseContext.cpp
index 75828189cd..97edf3ee7f 100644
--- a/src/compiler/translator/ParseContext.cpp
+++ b/src/compiler/translator/ParseContext.cpp
@@ -6750,6 +6750,37 @@ TIntermDeclaration *TParseContext::addInterfaceBlock(
         new TVariable(&symbolTable, instanceName, interfaceBlockType,
                       instanceName.empty() ? SymbolType::Empty : instanceSymbolType);
 
+    // Bandaid fix: enforce kWebGLMaxVariableSizeInBytes for interface blocks.
+    // addInterfaceBlock() creates the instance TVariable directly and never
+    // reaches declareVariable(), so the size check at line 2144 was structurally
+    // bypassed for UBOs. CalculateVariableSize() does not recurse into
+    // interface-block fields (only EbtStruct), so validate per-field here using
+    // the same gating as checkVariableSize(): WebGL only, vertex/fragment only,
+    // no prior errors. Each field is sized with std140 (the only layout WebGL
+    // permits for UBOs); a single oversized field is sufficient to reject.
+    if (mCompileOptions.rejectWebglShadersWithLargeVariables && numErrors() == 0 &&
+        (mShaderType == GL_VERTEX_SHADER || mShaderType == GL_FRAGMENT_SHADER))
+    {
+        angle::base::CheckedNumeric<size_t> blockSize = 0;
+        for (const TField *field : *fieldList)
+        {
+            blockSize += CalculateVariableSize(field->type(), /*isStd140=*/true);
+        }
+        if (arraySizes != nullptr)
+        {
+            for (unsigned int dim : *arraySizes)
+            {
+                blockSize *= dim;
+            }
+        }
+        if (blockSize.ValueOrDefault(std::numeric_limits<size_t>::max()) >
+            kWebGLMaxVariableSizeInBytes)
+        {
+            error(nameLine, "Size of declared variable exceeds implementation-defined limit",
+                  instanceName.empty() ? blockName : instanceName);
+        }
+    }
+
     checkVariableLocations(nameLine, instanceVariable);
     declareIRVariable(instanceVariable, sized);