diff --git a/qualtran/__init__.py b/qualtran/__init__.py
index 861928f7e..9ef7e03b2 100644
--- a/qualtran/__init__.py
+++ b/qualtran/__init__.py
@@ -64,6 +64,8 @@
     CInt,
     QIntOnesComp,
     CIntOnesComp,
+    QIntSignMag,
+    CIntSignMag,
     QUInt,
     CUInt,
     BQUInt,
diff --git a/qualtran/dtype/__init__.py b/qualtran/dtype/__init__.py
index 0f557b976..4510581dd 100644
--- a/qualtran/dtype/__init__.py
+++ b/qualtran/dtype/__init__.py
@@ -27,6 +27,8 @@
 
 from ._int_ones_complement import QIntOnesComp, CIntOnesComp
 
+from ._int_signmag import QIntSignMag, CIntSignMag
+
 from ._buint import BQUInt, BCUInt
 
 from ._fxp import QFxp, CFxp
diff --git a/qualtran/dtype/_int_signmag.py b/qualtran/dtype/_int_signmag.py
new file mode 100644
index 000000000..c28ad3b09
--- /dev/null
+++ b/qualtran/dtype/_int_signmag.py
@@ -0,0 +1,133 @@
+#  Copyright 2026 Google LLC
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+from functools import cached_property
+from typing import Iterable, List, Sequence
+
+import attrs
+import numpy as np
+from numpy.typing import NDArray
+
+from qualtran.symbolics import is_symbolic, SymbolicInt
+
+from ._base import BitEncoding, CDType, QDType
+
+
+@attrs.frozen
+class _IntSignMag(BitEncoding[int]):
+    """Sign-magnitude encoding.
+
+    The most significant bit is the sign bit (0=positive, 1=negative).
+    The remaining bits encode the absolute value.
+    """
+
+    bitsize: SymbolicInt
+
+    def get_domain(self) -> Iterable[int]:
+        max_val = 1 << (self.bitsize - 1)
+        return range(-max_val + 1, max_val)
+
+    def to_bits(self, x: int) -> List[int]:
+        if is_symbolic(self.bitsize):
+            raise ValueError(f"cannot compute bits with symbolic {self.bitsize=}")
+        self.assert_valid_val(x)
+        return [1 if x < 0 else 0] + [
+            int(b) for b in np.binary_repr(np.abs(x), width=self.bitsize - 1)
+        ]
+
+    def from_bits(self, bits: Sequence[int]) -> int:
+        sign = bits[0]
+        if self.bitsize == 1:
+            return 0
+        magnitude = 0
+        for b in bits[1:]:
+            magnitude = (magnitude << 1) | b
+        return -magnitude if sign else magnitude
+
+    def assert_valid_val(self, val: int, debug_str: str = 'val'):
+        if not isinstance(val, (int, np.integer)):
+            raise ValueError(f"{debug_str} should be an integer, not {val!r}")
+        max_val = 1 << (self.bitsize - 1)
+        if val <= -max_val:
+            raise ValueError(f"Too-small classical {self}: {val} encountered in {debug_str}")
+        if val >= max_val:
+            raise ValueError(f"Too-large classical {self}: {val} encountered in {debug_str}")
+
+    def assert_valid_val_array(self, val_array: NDArray[np.integer], debug_str: str = 'val'):
+        max_val = 1 << (self.bitsize - 1)
+        if np.any(val_array <= -max_val):
+            raise ValueError(f"Too-small classical {self}s encountered in {debug_str}")
+        if np.any(val_array >= max_val):
+            raise ValueError(f"Too-large classical {self}s encountered in {debug_str}")
+
+
+@attrs.frozen
+class QIntSignMag(QDType[int]):
+    """Sign-magnitude signed quantum integer.
+
+    The most significant bit is the sign bit (0=positive, 1=negative),
+    and the remaining bits encode the absolute value.
+
+    For an n-bit QSignInt, the representable range is [-(2^(n-1)-1), 2^(n-1)-1].
+    Note that this means +0 and -0 are distinct representations.
+
+    Args:
+        bitsize: The number of qubits used to represent the integer.
+    """
+
+    bitsize: SymbolicInt
+
+    def __attrs_post_init__(self):
+        if isinstance(self.bitsize, int):
+            if self.bitsize < 2:
+                raise ValueError("bitsize must be >= 2.")
+
+    @cached_property
+    def _bit_encoding(self) -> BitEncoding[int]:
+        return _IntSignMag(self.bitsize)
+
+    def is_symbolic(self) -> bool:
+        return is_symbolic(self.bitsize)
+
+    def __str__(self):
+        return f'QIntSignMag({self.bitsize})'
+
+
+@attrs.frozen
+class CIntSignMag(CDType[int]):
+    """Sign-magnitude signed classical integer.
+
+    The most significant bit is the sign bit (0=positive, 1=negative),
+    and the remaining bits encode the absolute value.
+
+    Args:
+        bitsize: The number of qubits used to represent the integer.
+    """
+
+    bitsize: SymbolicInt
+
+    def __attrs_post_init__(self):
+        if isinstance(self.bitsize, int):
+            if self.bitsize < 2:
+                raise ValueError("bitsize must be >= 2.")
+
+    @cached_property
+    def _bit_encoding(self) -> BitEncoding[int]:
+        return _IntSignMag(self.bitsize)
+
+    def is_symbolic(self) -> bool:
+        return is_symbolic(self.bitsize)
+
+    def __str__(self):
+        return f'CIntSignMag({self.bitsize})'
diff --git a/qualtran/dtype/_int_signmag_test.py b/qualtran/dtype/_int_signmag_test.py
new file mode 100644
index 000000000..ce890abe0
--- /dev/null
+++ b/qualtran/dtype/_int_signmag_test.py
@@ -0,0 +1,83 @@
+#  Copyright 2026 Google LLC
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+
+import pytest
+
+from qualtran.dtype import QIntSignMag
+
+
+def test_basic_properties():
+    dtype = QIntSignMag(4)
+    assert dtype.num_qubits == 4
+    assert not dtype.is_symbolic()
+    assert str(dtype) == 'QIntSignMag(4)'
+
+
+def test_bitsize_validation():
+    with pytest.raises(ValueError, match="bitsize must be >= 2"):
+        QIntSignMag(1)
+
+
+def test_to_bits_positive():
+    dtype = QIntSignMag(4)
+    assert dtype.to_bits(5) == [0, 1, 0, 1]
+    assert dtype.to_bits(0) == [0, 0, 0, 0]
+    assert dtype.to_bits(7) == [0, 1, 1, 1]
+
+
+def test_to_bits_negative():
+    dtype = QIntSignMag(4)
+    assert dtype.to_bits(-5) == [1, 1, 0, 1]
+    assert dtype.to_bits(-1) == [1, 0, 0, 1]
+    assert dtype.to_bits(-7) == [1, 1, 1, 1]
+
+
+def test_from_bits():
+    dtype = QIntSignMag(4)
+    assert dtype.from_bits([0, 1, 0, 1]) == 5
+    assert dtype.from_bits([1, 1, 0, 1]) == -5
+    assert dtype.from_bits([0, 0, 0, 0]) == 0
+    assert dtype.from_bits([1, 0, 0, 0]) == 0  # -0 == 0
+
+
+def test_roundtrip():
+    # to_bits -> from_bits should be identity for all valid values.
+    dtype = QIntSignMag(4)
+    for val in dtype.get_classical_domain():
+        bits = dtype.to_bits(val)
+        recovered = dtype.from_bits(bits)
+        assert recovered == val, f"Failed for {val}: bits={bits}, recovered={recovered}"
+
+
+def test_classical_domain():
+    dtype = QIntSignMag(4)
+    domain = list(dtype.get_classical_domain())
+    # 4-bit sign-magnitude: range is [-7, 7]
+    assert domain == list(range(-7, 8))
+
+
+def test_valid_classical_val():
+    dtype = QIntSignMag(4)
+    dtype.assert_valid_classical_val(0)
+    dtype.assert_valid_classical_val(7)
+    dtype.assert_valid_classical_val(-7)
+
+
+def test_invalid_classical_val():
+    dtype = QIntSignMag(4)
+    with pytest.raises(ValueError, match="Too-large"):
+        dtype.assert_valid_classical_val(8)
+    with pytest.raises(ValueError, match="Too-small"):
+        dtype.assert_valid_classical_val(-8)