diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index 1116940a4cc..66bd461157e 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -17,6 +17,8 @@ New Features
 - Added ``inherit='all_coords'`` option to :py:meth:`DataTree.to_dataset` to inherit
   all parent coordinates, not just indexed ones (:issue:`10812`, :pull:`11230`).
   By `Alfonso Ladino <https://github.com/aladinor>`_.
+- Added complex dtype support to FillValueCoder for the Zarr backend. (:pull:`11151`)
+  By `Max Jones <https://github.com/maxrjones>`_.
 
 Breaking Changes
 ~~~~~~~~~~~~~~~~
diff --git a/properties/test_encode_decode.py b/properties/test_encode_decode.py
index 87bbfdba933..bce2fc215c6 100644
--- a/properties/test_encode_decode.py
+++ b/properties/test_encode_decode.py
@@ -15,6 +15,7 @@
 import hypothesis.extra.numpy as npst
 import numpy as np
 from hypothesis import given
+from hypothesis import strategies as st
 
 import xarray as xr
 from xarray.coding.times import _parse_iso8601
@@ -48,6 +49,22 @@ def test_CFScaleOffset_coder_roundtrip(original) -> None:
     xr.testing.assert_identical(original, roundtripped)
 
 
+@given(
+    real=st.floats(allow_nan=True, allow_infinity=True),
+    imag=st.floats(allow_nan=True, allow_infinity=True),
+    dtype=st.sampled_from([np.complex64, np.complex128]),
+)
+def test_FillValueCoder_complex_roundtrip(real, imag, dtype) -> None:
+    from xarray.backends.zarr import FillValueCoder
+
+    value = dtype(complex(real, imag))
+    encoded = FillValueCoder.encode(value, np.dtype(dtype))
+    decoded = FillValueCoder.decode(encoded, np.dtype(dtype))
+    np.testing.assert_equal(
+        np.array(decoded, dtype=dtype), np.array(value, dtype=dtype)
+    )
+
+
 @given(dt=datetimes())
 def test_iso8601_decode(dt):
     iso = dt.isoformat()
diff --git a/xarray/backends/zarr.py b/xarray/backends/zarr.py
index 6681673025c..d9279dc2de9 100644
--- a/xarray/backends/zarr.py
+++ b/xarray/backends/zarr.py
@@ -122,40 +122,89 @@ class FillValueCoder:
     """
 
     @classmethod
-    def encode(cls, value: int | float | str | bytes, dtype: np.dtype[Any]) -> Any:
+    def encode(
+        cls, value: int | float | complex | str | bytes, dtype: np.dtype[Any]
+    ) -> Any:
         if dtype.kind == "S":
             # byte string, this implies that 'value' must also be `bytes` dtype.
-            assert isinstance(value, bytes)
+            if not isinstance(value, bytes):
+                raise TypeError(
+                    f"Failed to encode fill_value: expected bytes for dtype {dtype}, got {type(value).__name__}"
+                )
             return base64.standard_b64encode(value).decode()
         elif dtype.kind == "b":
             # boolean
             return bool(value)
         elif dtype.kind in "iu":
-            # todo: do we want to check for decimals?
+            if not isinstance(value, int | float | np.integer | np.floating):
+                raise TypeError(
+                    f"Failed to encode fill_value: expected int or float for dtype {dtype}, got {type(value).__name__}"
+                )
             return int(value)
         elif dtype.kind == "f":
+            if not isinstance(value, int | float | np.integer | np.floating):
+                raise TypeError(
+                    f"Failed to encode fill_value: expected int or float for dtype {dtype}, got {type(value).__name__}"
+                )
             return base64.standard_b64encode(struct.pack("<d", float(value))).decode()
+        elif dtype.kind == "c":
+            # complex - encode each component as base64, matching float encoding
+            if not isinstance(value, complex) and not np.issubdtype(
+                type(value), np.complexfloating
+            ):
+                raise TypeError(
+                    f"Failed to encode fill_value: expected complex for dtype {dtype}, got {type(value).__name__}"
+                )
+            return [
+                base64.standard_b64encode(
+                    struct.pack("<d", float(value.real))  # type: ignore[union-attr]
+                ).decode(),
+                base64.standard_b64encode(
+                    struct.pack("<d", float(value.imag))  # type: ignore[union-attr]
+                ).decode(),
+            ]
         elif dtype.kind == "U":
             return str(value)
         else:
             raise ValueError(f"Failed to encode fill_value. Unsupported dtype {dtype}")
 
     @classmethod
-    def decode(cls, value: int | float | str | bytes, dtype: str | np.dtype[Any]):
+    def decode(
+        cls, value: int | float | str | bytes | list, dtype: str | np.dtype[Any]
+    ):
         if dtype == "string":
             # zarr V3 string type
             return str(value)
         elif dtype == "bytes":
             # zarr V3 bytes type
-            assert isinstance(value, str | bytes)
+            if not isinstance(value, str | bytes):
+                raise TypeError(
+                    f"Failed to decode fill_value: expected str or bytes for dtype {dtype}, got {type(value).__name__}"
+                )
             return base64.standard_b64decode(value)
         np_dtype = np.dtype(dtype)
         if np_dtype.kind == "f":
-            assert isinstance(value, str | bytes)
+            if not isinstance(value, str | bytes):
+                raise TypeError(
+                    f"Failed to decode fill_value: expected str or bytes for dtype {np_dtype}, got {type(value).__name__}"
+                )
             return struct.unpack("<d", base64.standard_b64decode(value))[0]
+        elif np_dtype.kind == "c":
+            # complex - decode each component from base64, matching float decoding
+            if not (isinstance(value, list | tuple) and len(value) == 2):
+                raise TypeError(
+                    f"Failed to decode fill_value: expected a 2-element list for dtype {np_dtype}, got {type(value).__name__}"
+                )
+            real = struct.unpack("<d", base64.standard_b64decode(value[0]))[0]
+            imag = struct.unpack("<d", base64.standard_b64decode(value[1]))[0]
+            return complex(real, imag)
         elif np_dtype.kind == "b":
             return bool(value)
         elif np_dtype.kind in "iu":
+            if not isinstance(value, int | float | np.integer | np.floating):
+                raise TypeError(
+                    f"Failed to decode fill_value: expected int or float for dtype {np_dtype}, got {type(value).__name__}"
+                )
             return int(value)
         else:
             raise ValueError(f"Failed to decode fill_value. Unsupported dtype {dtype}")
diff --git a/xarray/tests/test_backends.py b/xarray/tests/test_backends.py
index e5b4acc894c..e42bfc2cd9f 100644
--- a/xarray/tests/test_backends.py
+++ b/xarray/tests/test_backends.py
@@ -7241,6 +7241,41 @@ def test_encode_zarr_attr_value() -> None:
     assert actual3 == expected3
 
 
+@requires_zarr
+@pytest.mark.parametrize("dtype", [complex, np.complex64, np.complex128])
+def test_fill_value_coder_complex(dtype) -> None:
+    """Test that FillValueCoder round-trips complex fill values."""
+    from xarray.backends.zarr import FillValueCoder
+
+    for value in [dtype(1 + 2j), dtype(-3.5 + 4.5j), dtype(complex("nan+nanj"))]:
+        encoded = FillValueCoder.encode(value, np.dtype(dtype))
+        decoded = FillValueCoder.decode(encoded, np.dtype(dtype))
+        np.testing.assert_equal(np.array(decoded, dtype=dtype), np.array(value))
+
+
+@requires_zarr
+@pytest.mark.parametrize(
+    "value,dtype",
+    [
+        (np.float32(np.inf), np.float32),
+        (np.float32(-np.inf), np.float32),
+        (np.float64(np.inf), np.float64),
+        (np.float64(-np.inf), np.float64),
+        (np.float32(np.nan), np.float32),
+        (np.float64(np.nan), np.float64),
+    ],
+)
+def test_fill_value_coder_inf_nan(value, dtype) -> None:
+    """Test that FillValueCoder round-trips inf and nan fill values."""
+    from xarray.backends.zarr import FillValueCoder
+
+    encoded = FillValueCoder.encode(value, np.dtype(dtype))
+    decoded = FillValueCoder.decode(encoded, np.dtype(dtype))
+    np.testing.assert_equal(
+        np.array(decoded, dtype=dtype), np.array(value, dtype=dtype)
+    )
+
+
 @requires_zarr
 def test_extract_zarr_variable_encoding() -> None:
     var = xr.Variable("x", [1, 2])