diff --git a/src/axolotl/loaders/model.py b/src/axolotl/loaders/model.py
index 6c8885526..eb5c087c4 100644
--- a/src/axolotl/loaders/model.py
+++ b/src/axolotl/loaders/model.py
@@ -173,6 +173,14 @@ class ModelLoader:
         PLUGIN_MANAGER.pre_model_load(self.cfg)
         self.patch_manager.apply_post_plugin_pre_model_load_patches()
         skip_move_to_device = self._build_model()
+
+        # Quantize 3D MoE expert nn.Parameter tensors that BnB skips during loading.
+        self.model._moe_experts_quantized = False
+        if self.cfg.adapter in ("qlora", "lora") and self.cfg.load_in_4bit:
+            from axolotl.monkeypatch.moe_quant import quantize_moe_expert_params
+
+            self.model._moe_experts_quantized = quantize_moe_expert_params(self.model)
+
         PLUGIN_MANAGER.post_model_build(self.cfg, self.model)
 
         # Post-build model configuration
@@ -860,6 +868,10 @@ class ModelLoader:
             # Make sure everything is in the same dtype
             skip_prepare_model_for_kbit_training = True
 
+        if getattr(self.model, "_moe_experts_quantized", False):
+            # Parametrized expert tensors dequantize on access — would OOM.
+            skip_prepare_model_for_kbit_training = True
+
         if (
             not skip_prepare_model_for_kbit_training
             and self.cfg.adapter in ["lora", "qlora"]
diff --git a/src/axolotl/monkeypatch/moe_quant.py b/src/axolotl/monkeypatch/moe_quant.py
new file mode 100644
index 000000000..c9a03e221
--- /dev/null
+++ b/src/axolotl/monkeypatch/moe_quant.py
@@ -0,0 +1,86 @@
+"""
+Post-load quantization for MoE expert weights stored as 3D nn.Parameter tensors.
+
+In transformers v5, many MoE models store expert weights as fused 3D nn.Parameter
+tensors instead of individual nn.Linear modules. BnB 4-bit quantization only targets
+nn.Linear, so these expert weights are skipped during model loading, causing OOM.
+
+This module provides a post-load fixup that quantizes those skipped parameters using
+bitsandbytes.nn.parametrize.replace_parameter_4bit (requires bitsandbytes >= 0.48.0).
+PEFT's target_parameters / ParamWrapper can then apply LoRA on top of these quantized
+params via stacked parametrizations.
+"""
+
+import bitsandbytes as bnb
+import torch
+
+from axolotl.utils.logging import get_logger
+
+LOG = get_logger(__name__)
+
+
+def find_unquantized_expert_params(model):
+    """Find 3D+ nn.Parameter tensors that BnB quantization skipped.
+
+    Returns:
+        List of (module, param_name) tuples to quantize.
+    """
+    params_to_quantize = []
+    for _, module in model.named_modules():
+        if isinstance(module, (bnb.nn.Linear4bit, bnb.nn.Linear8bitLt)):
+            continue
+        for param_name, param in module.named_parameters(recurse=False):
+            if param.ndim >= 3 and any(
+                kw in param_name for kw in ("experts", "gate_up_proj", "down_proj")
+            ):
+                params_to_quantize.append((module, param_name))
+    return params_to_quantize
+
+
+def quantize_moe_expert_params(model, quant_type=None, compress_statistics=None):
+    """Quantize 3D nn.Parameter expert weights that BnB skips during model loading.
+
+    Reads quant_type and compress_statistics from the model's quantization_config
+    when not explicitly provided, so that the same settings used for nn.Linear
+    quantization are applied to the MoE expert parameters.
+    """
+    from bitsandbytes.nn.parametrize import replace_parameter_4bit
+
+    params_to_quantize = find_unquantized_expert_params(model)
+    if not params_to_quantize:
+        return False
+
+    # Derive settings from model's BnB config if not explicitly provided
+    if quant_type is None or compress_statistics is None:
+        bnb_config = getattr(model.config, "quantization_config", None)
+        if bnb_config is not None:
+            if quant_type is None:
+                quant_type = getattr(bnb_config, "bnb_4bit_quant_type", "nf4")
+            if compress_statistics is None:
+                compress_statistics = getattr(
+                    bnb_config, "bnb_4bit_use_double_quant", True
+                )
+    # Final defaults
+    if quant_type is None:
+        quant_type = "nf4"
+    if compress_statistics is None:
+        compress_statistics = True
+
+    count = 0
+    for module, param_name in params_to_quantize:
+        replace_parameter_4bit(
+            module,
+            param_name,
+            compress_statistics=compress_statistics,
+            quant_type=quant_type,
+        )
+        count += 1
+
+    torch.cuda.empty_cache()
+    LOG.info(
+        "Quantized %d MoE expert parameters to 4-bit (quant_type=%s, compress_statistics=%s)",
+        count,
+        quant_type,
+        compress_statistics,
+    )
+    return True