diff --git a/deepspeed/runtime/engine.py b/deepspeed/runtime/engine.py
index 86918bd71c5a..2932335b09bb 100644
--- a/deepspeed/runtime/engine.py
+++ b/deepspeed/runtime/engine.py
@@ -2123,6 +2123,7 @@ def _configure_optimizer(self, client_optimizer, model_parameters):
         log_dist(f"DeepSpeed Basic Optimizer = {basic_optimizer.__class__.__name__}", ranks=[0])
 
         optimizer_wrapper = self._do_optimizer_sanity_check(basic_optimizer)
+        self._check_muon_can_reach_its_parameters(basic_optimizer, optimizer_wrapper)
 
         if optimizer_wrapper == ZERO_OPTIMIZATION:
             self.optimizer = self._configure_zero_optimizer(basic_optimizer)
@@ -2147,6 +2148,27 @@ def _configure_optimizer(self, client_optimizer, model_parameters):
         self.compression_scheduler = self._configure_compression_scheduler()
         self.quantizer = self._configure_quantization()
 
+    def _check_muon_can_reach_its_parameters(self, basic_optimizer, optimizer_wrapper):
+        """Refuse the one wrapper that hands Muon flat partitions and does not orthogonalize them.
+
+        `MuonWithAuxAdam.step` tells the two cases apart by shape: a matrix is the weight itself
+        and is orthogonalized there, a 1-D tensor is a ZeRO partition whose update the ZeRO
+        optimizer already applied. `BF16_Optimizer` breaks that reading - it replaces the param
+        groups with flat fp32 partitions (`param_group['params'] = [self.fp32_groups_flat_partition[i]]`)
+        and knows nothing about `use_muon`, so the update is never applied and the step is SGD.
+
+        The original shapes are not recoverable from `step`, so this is a refusal rather than a
+        fix; implementing Muon inside BF16_Optimizer is its own change. Reached by bf16 with
+        `grad_accum_dtype: fp32` at ZeRO stage 1.
+        """
+        if not isinstance(basic_optimizer, MuonWithAuxAdam) or optimizer_wrapper != BFLOAT16:
+            return
+        raise ZeRORuntimeException(
+            "Muon cannot be used with the BF16_Optimizer, which this configuration selects: bf16 "
+            "with grad_accum_dtype fp32 at ZeRO stage 1. That optimizer hands Muon flat fp32 "
+            "partitions and never applies the Newton-Schulz update, so training would silently "
+            "proceed as SGD. Drop grad_accum_dtype, or use ZeRO stage 2 or 3.")
+
     def _configure_autoep_folding_optimizer_gradient_reduction(self):
         configure = getattr(self.optimizer, "configure_autoep_folding_tp_gradient_reduction", None)
         if configure is None:
diff --git a/deepspeed/runtime/zero/muon/muon_optimizer.py b/deepspeed/runtime/zero/muon/muon_optimizer.py
index 3f199aeb97ea..ce129fd1c1a4 100644
--- a/deepspeed/runtime/zero/muon/muon_optimizer.py
+++ b/deepspeed/runtime/zero/muon/muon_optimizer.py
@@ -8,6 +8,7 @@
 try:
     from deepspeed.runtime.zero.muon.original_muon import MuonWithAuxAdam as BaseMuonWithAuxAdam
     from deepspeed.runtime.zero.muon.original_muon import adam_update
+    from deepspeed.runtime.zero.muon.original_muon import muon_update
 except ImportError:
     pass
 
@@ -60,11 +61,27 @@ def step(self, closure=None, step_id=None):
                 loss = closure()
         for group in self.param_groups:
             if group["use_muon"]:
-                # we move the muon update part to the deepspeed's optimizer since the parameter here is a flat version
-                # thus not suitable for muon update
                 for p in group["params"]:
+                    if p.grad is None:
+                        p.grad = torch.zeros_like(p)  # force synchronization
+                    if p.dim() < 2:
+                        # A flat ZeRO partition. ZeRO 1/2 orthogonalizes in get_flat_partition and
+                        # ZeRO-3 in its sub-group loop, so the gradient already holds the update
+                        # and only the weight decay and step size are left to apply.
+                        update = p.grad
+                    else:
+                        # The weight itself, so nothing has orthogonalized it: no ZeRO optimizer is
+                        # in play. Muon has to run here or the step degenerates to SGD.
+                        state = self.state[p]
+                        if len(state) == 0:
+                            state["momentum_buffer"] = torch.zeros_like(p)
+                        update = muon_update(p.grad,
+                                             state["momentum_buffer"],
+                                             beta=group["momentum"],
+                                             ns_method=group.get("ns_method", "gram"),
+                                             is_expert_group=getattr(p, "is_expert_group", False))
                     p.mul_(1 - group["lr"] * group["weight_decay"])
-                    p.add_(p.grad.reshape(p.shape), alpha=-group["lr"])
+                    p.add_(update.reshape(p.shape), alpha=-group["lr"])
 
         aux_param_groups = [group for group in self.param_groups if not group["use_muon"]]
         if self.aux_optimizer is not None:
diff --git a/tests/unit/runtime/zero/test_muon_without_zero_optimizer.py b/tests/unit/runtime/zero/test_muon_without_zero_optimizer.py
new file mode 100644
index 000000000000..5b6eede9a79f
--- /dev/null
+++ b/tests/unit/runtime/zero/test_muon_without_zero_optimizer.py
@@ -0,0 +1,183 @@
+# SPDX-License-Identifier: Apache-2.0
+
+# DeepSpeed Team
+"""Newton-Schulz has to run whether or not a ZeRO optimizer is there to do it.
+
+`MuonWithAuxAdam.step` applied an update it assumed had been orthogonalized already, which holds
+under ZeRO - the parameters it sees there are flat partitions and `get_flat_partition` or the
+ZeRO-3 sub-group loop did the work. At stage 0, the default, no ZeRO optimizer exists to have
+done it, and applying a raw gradient is SGD. Training runs and the loss falls, which is why
+counting the Newton-Schulz calls is the assertion that means something here.
+"""
+
+import contextlib
+
+import pytest
+import torch
+
+import deepspeed
+from deepspeed.accelerator import get_accelerator
+import deepspeed.runtime.zero.muon.original_muon as original_muon
+from deepspeed.runtime.zero.utils import ZeRORuntimeException
+from unit.common import DistributedTest
+
+NS_KERNELS = ("zeropower_via_gram_newtonschulz", "zeropower_via_newtonschulz5")
+
+
+@contextlib.contextmanager
+def counting_newton_schulz():
+    """Counts every Newton-Schulz call, whichever kernel the config selects.
+
+    Patched inside the test body rather than in a fixture: `DistributedTest` runs the body in a
+    worker process that a fixture in the parent would not reach.
+    """
+    calls = []
+    originals = {name: getattr(original_muon, name) for name in NS_KERNELS}
+
+    def counted(kernel):
+
+        def wrapper(*args, **kwargs):
+            calls.append(1)
+            return kernel(*args, **kwargs)
+
+        return wrapper
+
+    for name, kernel in originals.items():
+        setattr(original_muon, name, counted(kernel))
+    try:
+        yield calls
+    finally:
+        for name, kernel in originals.items():
+            setattr(original_muon, name, kernel)
+
+
+def _model():
+    return torch.nn.Sequential(torch.nn.Linear(32, 32, bias=False), torch.nn.Linear(32, 32, bias=False))
+
+
+def _config(stage, dtype="fp32"):
+    config = {
+        "train_micro_batch_size_per_gpu": 2,
+        "gradient_accumulation_steps": 1,
+        "gradient_clipping": 0.0,
+        "optimizer": {
+            "type": "Muon",
+            "params": {
+                "lr": 0.02
+            }
+        },
+    }
+    if stage is not None:
+        config["zero_optimization"] = {"stage": stage, "reduce_scatter": stage != 3}
+    if dtype != "fp32":
+        config[dtype] = {"enabled": True}
+        if dtype == "fp16":
+            config[dtype]["initial_scale_power"] = 4
+    return config
+
+
+def _skip_if_unsupported(dtype):
+    """Mirror the check the engine itself makes.
+
+    `_do_sanity_check` raises `Type fp16 is not supported on your device.` on
+    `not get_accelerator().is_fp16_supported()`, which is a different predicate from
+    `supported_dtypes()` -- the cpu-torch-latest runner reports fp16 in the latter and
+    False from the former, so guarding on the wrong one still fails there.
+    """
+    supported = {
+        "fp16": get_accelerator().is_fp16_supported,
+        "bf16": get_accelerator().is_bf16_supported,
+    }.get(dtype)
+    if supported is not None and not supported():
+        pytest.skip(f"{dtype} not supported on this accelerator")
+
+
+class TestMuonRunsWithoutAZeroOptimizer(DistributedTest):
+    world_size = 1
+
+    @pytest.mark.parametrize("dtype", ["fp32", "bf16", "fp16"])
+    def test_newton_schulz_runs_at_stage_zero(self, dtype):
+        """Every stage-0 wrapper: unwrapped for fp32, FP16_UnfusedOptimizer for bf16 and fp16.
+
+        Each hands `step` the weight itself rather than a flat partition, so nothing upstream has
+        orthogonalized it. On master all three do zero orthogonalizations and train as SGD.
+        """
+        _skip_if_unsupported(dtype)
+        model = _model()
+        engine, _, _, _ = deepspeed.initialize(model=model,
+                                               model_parameters=model.parameters(),
+                                               config=_config(0, dtype))
+
+        # Counting starts after initialize: FP16_UnfusedOptimizer steps once at construction to
+        # allocate state, and that call must not be what the assertion below is satisfied by.
+        with counting_newton_schulz() as calls:
+            x = torch.ones(2, 32, device=engine.device, dtype=next(engine.module.parameters()).dtype)
+            engine.backward(engine(x).square().sum())
+            engine.step()
+
+        assert len(calls) == 2, f"Newton-Schulz ran {len(calls)} times for two Muon matrices; expected one each"
+
+    def test_the_default_config_runs_muon(self):
+        """`zero_optimization.stage` defaults to 0, so this is the plainest Muon config there is."""
+        model = _model()
+        engine, _, _, _ = deepspeed.initialize(model=model, model_parameters=model.parameters(), config=_config(None))
+
+        with counting_newton_schulz() as calls:
+            x = torch.ones(2, 32, device=engine.device)
+            engine.backward(engine(x).square().sum())
+            engine.step()
+
+        assert len(calls) == 2, f"Newton-Schulz ran {len(calls)} times; the default config trained as SGD"
+
+    @pytest.mark.parametrize("stage", [1, 2, 3])
+    def test_newton_schulz_runs_on_the_supported_stages(self, stage):
+        """The positive control, and the assertion the existing tests were missing.
+
+        They check that training progresses, which SGD does too. Counting the orthogonalizations
+        is what distinguishes Muon from the update it degenerates to.
+        """
+        model = _model()
+        engine, _, _, _ = deepspeed.initialize(model=model, model_parameters=model.parameters(), config=_config(stage))
+
+        with counting_newton_schulz() as calls:
+            x = torch.ones(2, 32, device=engine.device, dtype=next(engine.module.parameters()).dtype)
+            engine.backward(engine(x).square().sum())
+            engine.step()
+
+        assert len(calls) == 2, \
+            f"Newton-Schulz ran {len(calls)} times for two Muon matrices; the step was not Muon"
+
+
+class TestMuonRefusesBF16Optimizer(DistributedTest):
+    """The one wrapper that hands Muon flat partitions without orthogonalizing them.
+
+    `BF16_Optimizer` replaces the param groups with flat fp32 partitions and knows nothing about
+    `use_muon`, so the shape test in `step` reads them as "ZeRO already did the update" and the
+    step is SGD. The original shapes are not recoverable there, so this is refused rather than
+    fixed. Selected by bf16 with `grad_accum_dtype: fp32` at ZeRO stage 1.
+    """
+    world_size = 1
+
+    def test_bf16_optimizer_with_muon_is_refused(self):
+        _skip_if_unsupported("bf16")
+        model = _model()
+        config = _config(1, "bf16")
+        config["data_types"] = {"grad_accum_dtype": "fp32"}
+
+        with pytest.raises(ZeRORuntimeException, match="BF16_Optimizer"):
+            deepspeed.initialize(model=model, model_parameters=model.parameters(), config=config)
+
+    def test_the_same_config_without_grad_accum_dtype_still_runs_muon(self):
+        """The neighbouring config, so the refusal is shown to be narrow."""
+        _skip_if_unsupported("bf16")
+        model = _model()
+        engine, _, _, _ = deepspeed.initialize(model=model,
+                                               model_parameters=model.parameters(),
+                                               config=_config(1, "bf16"))
+
+        with counting_newton_schulz() as calls:
+            x = torch.ones(2, 32, device=engine.device, dtype=next(engine.module.parameters()).dtype)
+            engine.backward(engine(x).square().sum())
+            engine.step()
+
+        assert len(calls) == 2