diff --git a/tests/kernels/moe/test_marlin_block_size_policy.py b/tests/kernels/moe/test_marlin_block_size_policy.py
new file mode 100644
index 000000000000..22cb8f3173c6
--- /dev/null
+++ b/tests/kernels/moe/test_marlin_block_size_policy.py
@@ -0,0 +1,66 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import torch
+
+from vllm.model_executor.layers.fused_moe.fused_marlin_moe import (
+    _choose_marlin_block_size_m,
+)
+from vllm.platforms.interface import DeviceCapability
+from vllm.scalar_type import scalar_types
+
+
+def test_gpt_oss_sm89_small_m_uses_decode_like_block_size() -> None:
+    block_size_m, policy = _choose_marlin_block_size_m(
+        num_tokens=1,
+        num_experts=32,
+        topk=4,
+        hidden_size=2880,
+        quant_type=scalar_types.float4_e2m1f,
+        input_dtype=None,
+        device_capability=DeviceCapability(8, 9),
+    )
+
+    assert (block_size_m, policy) == (64, "gpt_oss_sm89_decode_like")
+
+
+def test_gpt_oss_sm89_large_m_uses_prefill_block_size() -> None:
+    block_size_m, policy = _choose_marlin_block_size_m(
+        num_tokens=1024,
+        num_experts=32,
+        topk=4,
+        hidden_size=2880,
+        quant_type=scalar_types.float4_e2m1f,
+        input_dtype=None,
+        device_capability=DeviceCapability(8, 9),
+    )
+
+    assert (block_size_m, policy) == (32, "gpt_oss_sm89_prefill_like")
+
+
+def test_non_sm89_gpt_oss_shape_uses_generic_policy() -> None:
+    block_size_m, policy = _choose_marlin_block_size_m(
+        num_tokens=1,
+        num_experts=32,
+        topk=4,
+        hidden_size=2880,
+        quant_type=scalar_types.float4_e2m1f,
+        input_dtype=None,
+        device_capability=DeviceCapability(9, 0),
+    )
+
+    assert (block_size_m, policy) == (8, "auto")
+
+
+def test_generic_auto_policy_keeps_int8_floor() -> None:
+    block_size_m, policy = _choose_marlin_block_size_m(
+        num_tokens=16,
+        num_experts=32,
+        topk=1,
+        hidden_size=4096,
+        quant_type=scalar_types.uint4,
+        input_dtype=torch.int8,
+        device_capability=DeviceCapability(8, 9),
+    )
+
+    assert (block_size_m, policy) == (16, "auto")
diff --git a/vllm/model_executor/layers/fused_moe/fused_marlin_moe.py b/vllm/model_executor/layers/fused_moe/fused_marlin_moe.py
index 136a8188d6a0..d1300467025d 100644
--- a/vllm/model_executor/layers/fused_moe/fused_marlin_moe.py
+++ b/vllm/model_executor/layers/fused_moe/fused_marlin_moe.py
@@ -44,8 +44,70 @@
     kNvfp4Static,
 )
 from vllm.platforms import current_platform
+from vllm.platforms.interface import DeviceCapability
 from vllm.scalar_type import ScalarType, scalar_types
 
+GPT_OSS_SM89_MOE_BLOCK_SIZE_M_SMALL_M = 64
+GPT_OSS_SM89_MOE_BLOCK_SIZE_M_LARGE_M = 32
+GPT_OSS_SM89_MOE_SMALL_M_THRESHOLD = 128
+
+
+def _use_gpt_oss_sm89_marlin_block_size_policy(
+    *,
+    num_experts: int,
+    topk: int,
+    hidden_size: int,
+    quant_type: ScalarType,
+    device_capability: DeviceCapability | None,
+) -> bool:
+    return (
+        device_capability == DeviceCapability(8, 9)
+        and num_experts == 32
+        and topk == 4
+        and hidden_size == 2880
+        and quant_type == scalar_types.float4_e2m1f
+    )
+
+
+def _choose_marlin_block_size_m(
+    *,
+    num_tokens: int,
+    num_experts: int,
+    topk: int,
+    hidden_size: int,
+    quant_type: ScalarType,
+    input_dtype: torch.dtype | None,
+    device_capability: DeviceCapability | None,
+) -> tuple[int, str]:
+    # GPT-OSS on SM89/L40S benefits from a larger block during tiny-M
+    # decode-like calls and a smaller block during prefill-like calls. Keep
+    # this narrow to the observed GPT-OSS MXFP4 MoE problem shape.
+    if _use_gpt_oss_sm89_marlin_block_size_policy(
+        num_experts=num_experts,
+        topk=topk,
+        hidden_size=hidden_size,
+        quant_type=quant_type,
+        device_capability=device_capability,
+    ):
+        if num_tokens <= GPT_OSS_SM89_MOE_SMALL_M_THRESHOLD:
+            return (
+                GPT_OSS_SM89_MOE_BLOCK_SIZE_M_SMALL_M,
+                "gpt_oss_sm89_decode_like",
+            )
+        return (
+            GPT_OSS_SM89_MOE_BLOCK_SIZE_M_LARGE_M,
+            "gpt_oss_sm89_prefill_like",
+        )
+
+    for block_size_m in [8, 16, 32, 48, 64]:
+        if num_tokens * topk / num_experts / block_size_m < 0.9:
+            break
+
+    if input_dtype is not None and input_dtype.itemsize == 1:
+        block_size_m = max(block_size_m, 16)
+
+    return block_size_m, "auto"
+
 
 def _fused_marlin_moe(
     hidden_states: torch.Tensor,
@@ -304,14 +366,19 @@ def fused_marlin_moe(
     assert num_bits in [4, 8]
     assert topk_weights.dtype == torch.float32
 
-    # M block size selection logic
-    # TODO: tune this further for specific models
-    for block_size_m in [8, 16, 32, 48, 64]:
-        if M * topk / E / block_size_m < 0.9:
-            break
-
-    if input_dtype is not None and input_dtype.itemsize == 1:
-        block_size_m = max(block_size_m, 16)
+    block_size_m, _ = _choose_marlin_block_size_m(
+        num_tokens=M,
+        num_experts=E,
+        topk=topk,
+        hidden_size=K,
+        quant_type=quant_type,
+        input_dtype=input_dtype,
+        device_capability=(
+            current_platform.get_device_capability()
+            if current_platform.is_cuda()
+            else None
+        ),
+    )
 
     if global_num_experts == -1:
         global_num_experts = E