diff --git a/pandas/_libs/tslibs/__init__.py b/pandas/_libs/tslibs/__init__.py
index 7cbc1833093ba..b3a006141fadc 100644
--- a/pandas/_libs/tslibs/__init__.py
+++ b/pandas/_libs/tslibs/__init__.py
@@ -25,6 +25,7 @@
     "Tick",
     "BaseOffset",
     "tz_compare",
+    "is_unitless",
 ]
 
 from pandas._libs.tslibs import dtypes
@@ -39,6 +40,7 @@
 from pandas._libs.tslibs.np_datetime import (
     OutOfBoundsDatetime,
     OutOfBoundsTimedelta,
+    is_unitless,
 )
 from pandas._libs.tslibs.offsets import (
     BaseOffset,
diff --git a/pandas/_libs/tslibs/np_datetime.pyi b/pandas/_libs/tslibs/np_datetime.pyi
index 59f4427125266..27871a78f8aaf 100644
--- a/pandas/_libs/tslibs/np_datetime.pyi
+++ b/pandas/_libs/tslibs/np_datetime.pyi
@@ -9,3 +9,4 @@ def py_td64_to_tdstruct(td64: int, unit: int) -> dict: ...
 def astype_overflowsafe(
     arr: np.ndarray, dtype: np.dtype, copy: bool = ...
 ) -> np.ndarray: ...
+def is_unitless(dtype: np.dtype) -> bool: ...
diff --git a/pandas/_libs/tslibs/np_datetime.pyx b/pandas/_libs/tslibs/np_datetime.pyx
index 07d198193464f..a787bded2bd50 100644
--- a/pandas/_libs/tslibs/np_datetime.pyx
+++ b/pandas/_libs/tslibs/np_datetime.pyx
@@ -92,6 +92,18 @@ def py_get_unit_from_dtype(dtype):
     return get_unit_from_dtype(dtype)
 
 
+def is_unitless(dtype: cnp.dtype) -> bool:
+    """
+    Check if a datetime64 or timedelta64 dtype has no attached unit.
+    """
+    if dtype.type_num not in [cnp.NPY_DATETIME, cnp.NPY_TIMEDELTA]:
+        raise ValueError("is_unitless dtype must be datetime64 or timedelta64")
+    cdef:
+        NPY_DATETIMEUNIT unit = get_unit_from_dtype(dtype)
+
+    return unit == NPY_DATETIMEUNIT.NPY_FR_GENERIC
+
+
 # ----------------------------------------------------------------------
 # Comparison
 
diff --git a/pandas/_libs/tslibs/timedeltas.pyi b/pandas/_libs/tslibs/timedeltas.pyi
index a04104915cf1f..c547503bae273 100644
--- a/pandas/_libs/tslibs/timedeltas.pyi
+++ b/pandas/_libs/tslibs/timedeltas.pyi
@@ -88,6 +88,8 @@ class Timedelta(timedelta):
     # GH 46171
     # While Timedelta can return pd.NaT, having the constructor return
     # a Union with NaTType makes things awkward for users of pandas
+    @classmethod
+    def _from_value_and_reso(cls, value: np.int64, reso: int) -> Timedelta: ...
     @property
     def days(self) -> int: ...
     @property
diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index e7f97413f6881..7fef934a85626 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -31,6 +31,7 @@
     iNaT,
     ints_to_pydatetime,
     is_date_array_normalized,
+    is_unitless,
     normalize_i8_timestamps,
     timezones,
     to_offset,
@@ -335,7 +336,12 @@ def _simple_new(  # type: ignore[override]
         cls, values: np.ndarray, freq: BaseOffset | None = None, dtype=DT64NS_DTYPE
     ) -> DatetimeArray:
         assert isinstance(values, np.ndarray)
-        assert values.dtype == DT64NS_DTYPE
+        assert dtype.kind == "M"
+        if isinstance(dtype, np.dtype):
+            # TODO: once non-nano DatetimeTZDtype is implemented, require that
+            #  dtype's reso match values's reso
+            assert dtype == values.dtype
+            assert not is_unitless(dtype)
 
         result = super()._simple_new(values, dtype)
         result._freq = freq
@@ -761,7 +767,7 @@ def _add_offset(self, offset) -> DatetimeArray:
             else:
                 values = self
             result = offset._apply_array(values).view("M8[ns]")
-            result = DatetimeArray._simple_new(result)
+            result = DatetimeArray._simple_new(result, dtype=result.dtype)
             result = result.tz_localize(self.tz)
 
         except NotImplementedError:
diff --git a/pandas/core/arrays/timedeltas.py b/pandas/core/arrays/timedeltas.py
index 1f55842050df0..816f07b076ef8 100644
--- a/pandas/core/arrays/timedeltas.py
+++ b/pandas/core/arrays/timedeltas.py
@@ -29,6 +29,7 @@
     precision_from_unit,
 )
 from pandas._libs.tslibs.fields import get_timedelta_field
+from pandas._libs.tslibs.np_datetime import py_get_unit_from_dtype
 from pandas._libs.tslibs.timedeltas import (
     array_to_timedelta64,
     ints_to_pytimedelta,
@@ -40,6 +41,7 @@
     npt,
 )
 from pandas.compat.numpy import function as nv
+from pandas.util._decorators import cache_readonly
 from pandas.util._validators import validate_endpoints
 
 from pandas.core.dtypes.astype import astype_td64_unit_conversion
@@ -154,8 +156,15 @@ class TimedeltaArray(dtl.TimelikeOps):
     # Note: ndim must be defined to ensure NaT.__richcmp__(TimedeltaArray)
     #  operates pointwise.
 
+    @cache_readonly
+    def _reso(self):
+        return py_get_unit_from_dtype(self.dtype)
+
     def _box_func(self, x: np.timedelta64) -> Timedelta | NaTType:
-        return Timedelta(x, unit="ns")
+        y = x.view("i8")
+        if y == NaT.value:
+            return NaT
+        return Timedelta._from_value_and_reso(y, reso=self._reso)
 
     @property
     # error: Return type "dtype" of "dtype" incompatible with return type
@@ -174,7 +183,7 @@ def dtype(self) -> np.dtype:  # type: ignore[override]
         -------
         numpy.dtype
         """
-        return TD64NS_DTYPE
+        return self._ndarray.dtype
 
     # ----------------------------------------------------------------
     # Constructors
@@ -244,11 +253,13 @@ def __init__(
     def _simple_new(  # type: ignore[override]
         cls, values: np.ndarray, freq: BaseOffset | None = None, dtype=TD64NS_DTYPE
     ) -> TimedeltaArray:
-        assert dtype == TD64NS_DTYPE, dtype
+        # Require td64 dtype, not unit-less, matching values.dtype
+        assert isinstance(dtype, np.dtype) and dtype.kind == "m"
+        assert not tslibs.is_unitless(dtype)
         assert isinstance(values, np.ndarray), type(values)
-        assert values.dtype == TD64NS_DTYPE
+        assert dtype == values.dtype
 
-        result = super()._simple_new(values=values, dtype=TD64NS_DTYPE)
+        result = super()._simple_new(values=values, dtype=dtype)
         result._freq = freq
         return result
 
@@ -262,7 +273,7 @@ def _from_sequence(
         data, inferred_freq = sequence_to_td64ns(data, copy=copy, unit=None)
         freq, _ = dtl.validate_inferred_freq(None, inferred_freq, False)
 
-        return cls._simple_new(data, freq=freq)
+        return cls._simple_new(data, dtype=data.dtype, freq=freq)
 
     @classmethod
     def _from_sequence_not_strict(
@@ -286,7 +297,7 @@ def _from_sequence_not_strict(
         if explicit_none:
             freq = None
 
-        result = cls._simple_new(data, freq=freq)
+        result = cls._simple_new(data, dtype=data.dtype, freq=freq)
 
         if inferred_freq is None and freq is not None:
             # this condition precludes `freq_infer`
@@ -330,7 +341,8 @@ def _generate_range(cls, start, end, periods, freq, closed=None):
         if not right_closed:
             index = index[:-1]
 
-        return cls._simple_new(index.view("m8[ns]"), freq=freq)
+        td64values = index.view("m8[ns]")
+        return cls._simple_new(td64values, dtype=td64values.dtype, freq=freq)
 
     # ----------------------------------------------------------------
     # DatetimeLike Interface
diff --git a/pandas/tests/arrays/test_datetimes.py b/pandas/tests/arrays/test_datetimes.py
index 9ea87be2a5468..8eb5cc2dd82f6 100644
--- a/pandas/tests/arrays/test_datetimes.py
+++ b/pandas/tests/arrays/test_datetimes.py
@@ -11,6 +11,17 @@
 from pandas.core.arrays import DatetimeArray
 
 
+class TestNonNano:
+    @pytest.mark.parametrize("unit,reso", [("s", 7), ("ms", 8), ("us", 9)])
+    @pytest.mark.xfail(reason="_box_func is not yet patched to get reso right")
+    def test_non_nano(self, unit, reso):
+        arr = np.arange(5, dtype=np.int64).view(f"M8[{unit}]")
+        dta = DatetimeArray._simple_new(arr, dtype=arr.dtype)
+
+        assert dta.dtype == arr.dtype
+        assert dta[0]._reso == reso
+
+
 class TestDatetimeArrayComparisons:
     # TODO: merge this into tests/arithmetic/test_datetime64 once it is
     #  sufficiently robust
diff --git a/pandas/tests/arrays/test_timedeltas.py b/pandas/tests/arrays/test_timedeltas.py
index bf3491496ab3a..46306167878f6 100644
--- a/pandas/tests/arrays/test_timedeltas.py
+++ b/pandas/tests/arrays/test_timedeltas.py
@@ -7,6 +7,16 @@
 from pandas.core.arrays import TimedeltaArray
 
 
+class TestNonNano:
+    @pytest.mark.parametrize("unit,reso", [("s", 7), ("ms", 8), ("us", 9)])
+    def test_non_nano(self, unit, reso):
+        arr = np.arange(5, dtype=np.int64).view(f"m8[{unit}]")
+        tda = TimedeltaArray._simple_new(arr, dtype=arr.dtype)
+
+        assert tda.dtype == arr.dtype
+        assert tda[0]._reso == reso
+
+
 class TestTimedeltaArray:
     @pytest.mark.parametrize("dtype", [int, np.int32, np.int64, "uint32", "uint64"])
     def test_astype_int(self, dtype):
diff --git a/pandas/tests/tslibs/test_api.py b/pandas/tests/tslibs/test_api.py
index 9655bb88c2fcf..273a7985ff50b 100644
--- a/pandas/tests/tslibs/test_api.py
+++ b/pandas/tests/tslibs/test_api.py
@@ -50,6 +50,7 @@ def test_namespace():
         "tz_convert_from_utc_single",
         "to_offset",
         "tz_compare",
+        "is_unitless",
     ]
 
     expected = set(submodules + api)
diff --git a/pandas/tests/tslibs/test_np_datetime.py b/pandas/tests/tslibs/test_np_datetime.py
index 336c7d30d5f77..31f48b9ad7c4a 100644
--- a/pandas/tests/tslibs/test_np_datetime.py
+++ b/pandas/tests/tslibs/test_np_datetime.py
@@ -4,6 +4,7 @@
 from pandas._libs.tslibs.np_datetime import (
     OutOfBoundsDatetime,
     astype_overflowsafe,
+    is_unitless,
     py_get_unit_from_dtype,
     py_td64_to_tdstruct,
 )
@@ -11,6 +12,28 @@
 import pandas._testing as tm
 
 
+def test_is_unitless():
+    dtype = np.dtype("M8[ns]")
+    assert not is_unitless(dtype)
+
+    dtype = np.dtype("datetime64")
+    assert is_unitless(dtype)
+
+    dtype = np.dtype("m8[ns]")
+    assert not is_unitless(dtype)
+
+    dtype = np.dtype("timedelta64")
+    assert is_unitless(dtype)
+
+    msg = "dtype must be datetime64 or timedelta64"
+    with pytest.raises(ValueError, match=msg):
+        is_unitless(np.dtype(np.int64))
+
+    msg = "Argument 'dtype' has incorrect type"
+    with pytest.raises(TypeError, match=msg):
+        is_unitless("foo")
+
+
 def test_get_unit_from_dtype():
     # datetime64
     assert py_get_unit_from_dtype(np.dtype("M8[Y]")) == 0