diff --git a/pandas/_libs/tslib.pyx b/pandas/_libs/tslib.pyx
index c2d2acd95ce43..4201c499cce72 100644
--- a/pandas/_libs/tslib.pyx
+++ b/pandas/_libs/tslib.pyx
@@ -551,7 +551,7 @@ cpdef array_to_datetime(
                     continue
 
                 _ts = convert_str_to_tsobject(
-                    val, None, unit="ns", dayfirst=dayfirst, yearfirst=yearfirst
+                    val, None, dayfirst=dayfirst, yearfirst=yearfirst
                 )
                 item_reso = _ts.creso
                 state.update_creso(item_reso)
@@ -716,7 +716,7 @@ cdef _array_to_datetime_object(
 
             try:
                 tsobj = convert_str_to_tsobject(
-                    val, None, unit="ns", dayfirst=dayfirst, yearfirst=yearfirst
+                    val, None, dayfirst=dayfirst, yearfirst=yearfirst
                 )
                 tsobj.ensure_reso(NPY_FR_ns, val)
 
diff --git a/pandas/_libs/tslibs/conversion.pxd b/pandas/_libs/tslibs/conversion.pxd
index e97f4900d20c8..7ffd630d6d8e1 100644
--- a/pandas/_libs/tslibs/conversion.pxd
+++ b/pandas/_libs/tslibs/conversion.pxd
@@ -37,7 +37,7 @@ cdef _TSObject convert_datetime_to_tsobject(datetime ts, tzinfo tz,
                                             int32_t nanos=*,
                                             NPY_DATETIMEUNIT reso=*)
 
-cdef _TSObject convert_str_to_tsobject(str ts, tzinfo tz, str unit,
+cdef _TSObject convert_str_to_tsobject(str ts, tzinfo tz,
                                        bint dayfirst=*,
                                        bint yearfirst=*)
 
diff --git a/pandas/_libs/tslibs/conversion.pyx b/pandas/_libs/tslibs/conversion.pyx
index 89b420b18a980..036faea0cdc1f 100644
--- a/pandas/_libs/tslibs/conversion.pyx
+++ b/pandas/_libs/tslibs/conversion.pyx
@@ -281,7 +281,7 @@ cdef _TSObject convert_to_tsobject(object ts, tzinfo tz, str unit,
     obj = _TSObject()
 
     if isinstance(ts, str):
-        return convert_str_to_tsobject(ts, tz, unit, dayfirst, yearfirst)
+        return convert_str_to_tsobject(ts, tz, dayfirst, yearfirst)
 
     if checknull_with_nat_and_na(ts):
         obj.value = NPY_NAT
@@ -473,7 +473,7 @@ cdef _adjust_tsobject_tz_using_offset(_TSObject obj, tzinfo tz):
     check_overflows(obj, obj.creso)
 
 
-cdef _TSObject convert_str_to_tsobject(str ts, tzinfo tz, str unit,
+cdef _TSObject convert_str_to_tsobject(str ts, tzinfo tz,
                                        bint dayfirst=False,
                                        bint yearfirst=False):
     """
@@ -489,7 +489,6 @@ cdef _TSObject convert_str_to_tsobject(str ts, tzinfo tz, str unit,
         Value to be converted to _TSObject
     tz : tzinfo or None
         timezone for the timezone-aware output
-    unit : str or None
     dayfirst : bool, default False
         When parsing an ambiguous date string, interpret e.g. "3/4/1975" as
         April 3, as opposed to the standard US interpretation March 4.