diff --git a/tests/kernels/test_attention_selector.py b/tests/kernels/test_attention_selector.py
index a08c874407e..492acb91e8e 100644
--- a/tests/kernels/test_attention_selector.py
+++ b/tests/kernels/test_attention_selector.py
@@ -94,7 +94,12 @@ def test_flash_attn(monkeypatch):
 
 
 def test_invalid_env(monkeypatch):
-    """Throw an exception if the backend name is invalid."""
+    """Ignore the invalid env variable if it is set."""
     override_backend_env_variable(monkeypatch, STR_INVALID_VAL)
-    with pytest.raises(ValueError):
-        get_attn_backend(16, torch.float16, None, 16, False)
+    with patch("vllm.attention.selector.current_platform", CudaPlatform()):
+        backend = get_attn_backend(32, torch.float16, None, 16, False)
+        assert backend.get_name() == "FLASH_ATTN"
+
+        # when block size == 16, backend will fall back to XFORMERS
+        backend = get_attn_backend(16, torch.float16, None, 16, False)
+        assert backend.get_name() == "XFORMERS"
diff --git a/tests/plugins/vllm_add_dummy_platform/vllm_add_dummy_platform/dummy_attention_backend.py b/tests/plugins/vllm_add_dummy_platform/vllm_add_dummy_platform/dummy_attention_backend.py
new file mode 100644
index 00000000000..5634be3c8d8
--- /dev/null
+++ b/tests/plugins/vllm_add_dummy_platform/vllm_add_dummy_platform/dummy_attention_backend.py
@@ -0,0 +1,8 @@
+from vllm.attention.backends.flash_attn import FlashAttentionBackend
+
+
+class DummyAttentionBackend(FlashAttentionBackend):
+
+    @staticmethod
+    def get_name() -> str:
+        return "Dummy_Backend"
diff --git a/tests/plugins/vllm_add_dummy_platform/vllm_add_dummy_platform/dummy_platform.py b/tests/plugins/vllm_add_dummy_platform/vllm_add_dummy_platform/dummy_platform.py
index fde93142f11..84721d5971c 100644
--- a/tests/plugins/vllm_add_dummy_platform/vllm_add_dummy_platform/dummy_platform.py
+++ b/tests/plugins/vllm_add_dummy_platform/vllm_add_dummy_platform/dummy_platform.py
@@ -3,3 +3,7 @@
 
 class DummyPlatform(CudaPlatform):
     device_name = "DummyDevice"
+
+    def get_attn_backend_cls(self, backend_name, head_size, dtype,
+                             kv_cache_dtype, block_size, use_v1):
+        return "vllm_add_dummy_platform.dummy_attention_backend.DummyAttentionBackend"  # noqa E501
diff --git a/tests/plugins_tests/test_platform_plugins.py b/tests/plugins_tests/test_platform_plugins.py
index 69698b34c71..661aa5f649a 100644
--- a/tests/plugins_tests/test_platform_plugins.py
+++ b/tests/plugins_tests/test_platform_plugins.py
@@ -1,3 +1,10 @@
+import torch
+
+from tests.kernels.utils import override_backend_env_variable
+from vllm.attention.selector import get_attn_backend
+from vllm.utils import STR_INVALID_VAL
+
+
 def test_platform_plugins():
     # simulate workload by running an example
     import runpy
@@ -14,3 +21,10 @@ def test_platform_plugins():
         f"Expected DummyDevice, got {current_platform.device_name}, "
         "possibly because current_platform is imported before the plugin"
         f" is loaded. The first import:\n{_init_trace}")
+
+
+def test_oot_attention_backend(monkeypatch):
+    # ignore the backend env variable if it is set
+    override_backend_env_variable(monkeypatch, STR_INVALID_VAL)
+    backend = get_attn_backend(16, torch.float16, torch.float16, 16, False)
+    assert backend.get_name() == "Dummy_Backend"
diff --git a/vllm/attention/layer.py b/vllm/attention/layer.py
index a283e87d840..9b03fd73fe6 100644
--- a/vllm/attention/layer.py
+++ b/vllm/attention/layer.py
@@ -190,11 +190,11 @@ def __init__(
                                         kv_cache_dtype=None,
                                         block_size=16,
                                         is_attention_free=False)
-        attn_backend = backend_name_to_enum(attn_backend.get_name())
-        if attn_backend in {_Backend.FLASH_ATTN, _Backend.FLASH_ATTN_VLLM_V1}:
-            attn_backend = _Backend.XFORMERS
+        backend = backend_name_to_enum(attn_backend.get_name())
+        if backend in {_Backend.FLASH_ATTN, _Backend.FLASH_ATTN_VLLM_V1}:
+            backend = _Backend.XFORMERS
 
-        self.attn_backend = attn_backend if attn_backend in {
+        self.attn_backend = backend if backend in {
             _Backend.TORCH_SDPA, _Backend.XFORMERS
         } else _Backend.TORCH_SDPA
 
diff --git a/vllm/attention/selector.py b/vllm/attention/selector.py
index 0ff007c87b1..81ea6eefb54 100644
--- a/vllm/attention/selector.py
+++ b/vllm/attention/selector.py
@@ -14,16 +14,18 @@
 logger = init_logger(__name__)
 
 
-def backend_name_to_enum(backend_name: str) -> _Backend:
-    assert backend_name is not None
-
-    backend_members = _Backend.__members__
-    if backend_name not in backend_members:
-        raise ValueError(f"Invalid attention backend '{backend_name}'. "
-                         f"Available backends: {', '.join(backend_members)} "
-                         "(case-sensitive).")
+def backend_name_to_enum(backend_name: str) -> Optional[_Backend]:
+    """
+    Convert a string backend name to a _Backend enum value.
 
-    return _Backend[backend_name]
+    Returns:
+    * _Backend: enum value if backend_name is a valid in-tree type
+    * None: otherwise it's an invalid in-tree type or an out-of-tree platform is
+            loaded.
+    """
+    assert backend_name is not None
+    return _Backend[backend_name] if backend_name in _Backend.__members__ else \
+          None
 
 
 def get_env_variable_attn_backend() -> Optional[_Backend]: