diff --git a/pandas/_libs/hashtable.pxd b/pandas/_libs/hashtable.pxd
index 51ec4ba43159c..0499eabf708af 100644
--- a/pandas/_libs/hashtable.pxd
+++ b/pandas/_libs/hashtable.pxd
@@ -36,8 +36,8 @@ cdef class PyObjectHashTable(HashTable):
 cdef class StringHashTable(HashTable):
     cdef kh_str_t *table
 
-    cpdef get_item(self, object val)
-    cpdef set_item(self, object key, Py_ssize_t val)
+    cpdef get_item(self, str val)
+    cpdef set_item(self, str key, Py_ssize_t val)
 
 cdef struct Int64VectorData:
     int64_t *data
diff --git a/pandas/_libs/hashtable_class_helper.pxi.in b/pandas/_libs/hashtable_class_helper.pxi.in
index b207fcb66948d..7d57c67e70b58 100644
--- a/pandas/_libs/hashtable_class_helper.pxi.in
+++ b/pandas/_libs/hashtable_class_helper.pxi.in
@@ -599,7 +599,7 @@ cdef class StringHashTable(HashTable):
                                        sizeof(Py_ssize_t) + # vals
                                        sizeof(uint32_t)) # flags
 
-    cpdef get_item(self, object val):
+    cpdef get_item(self, str val):
         cdef:
             khiter_t k
             const char *v
@@ -611,16 +611,16 @@ cdef class StringHashTable(HashTable):
         else:
             raise KeyError(val)
 
-    cpdef set_item(self, object key, Py_ssize_t val):
+    cpdef set_item(self, str key, Py_ssize_t val):
         cdef:
             khiter_t k
             int ret = 0
             const char *v
 
-        v = get_c_string(val)
+        v = get_c_string(key)
 
         k = kh_put_str(self.table, v, &ret)
-        self.table.keys[k] = key
+        self.table.keys[k] = v
         if kh_exist_str(self.table, k):
             self.table.vals[k] = val
         else:
@@ -784,7 +784,7 @@ cdef class StringHashTable(HashTable):
                 labels[i] = na_sentinel
             else:
                 # if ignore_na is False, we also stringify NaN/None/etc.
-                v = get_c_string(val)
+                v = get_c_string(<str>val)
                 vecs[i] = v
 
         # compute
diff --git a/pandas/_libs/tslibs/util.pxd b/pandas/_libs/tslibs/util.pxd
index 63cbd36f9cd1d..936532a81c6d6 100644
--- a/pandas/_libs/tslibs/util.pxd
+++ b/pandas/_libs/tslibs/util.pxd
@@ -218,7 +218,7 @@ cdef inline bint is_nan(object val):
     return is_complex_object(val) and val != val
 
 
-cdef inline const char* get_c_string_buf_and_size(object py_string,
+cdef inline const char* get_c_string_buf_and_size(str py_string,
                                                   Py_ssize_t *length):
     """
     Extract internal char* buffer of unicode or bytes object `py_string` with
@@ -231,7 +231,7 @@ cdef inline const char* get_c_string_buf_and_size(object py_string,
 
     Parameters
     ----------
-    py_string : object
+    py_string : str
     length : Py_ssize_t*
 
     Returns
@@ -241,12 +241,9 @@ cdef inline const char* get_c_string_buf_and_size(object py_string,
     cdef:
         const char *buf
 
-    if PyUnicode_Check(py_string):
-        buf = PyUnicode_AsUTF8AndSize(py_string, length)
-    else:
-        PyBytes_AsStringAndSize(py_string, <char**>&buf, length)
+    buf = PyUnicode_AsUTF8AndSize(py_string, length)
     return buf
 
 
-cdef inline const char* get_c_string(object py_string):
+cdef inline const char* get_c_string(str py_string):
     return get_c_string_buf_and_size(py_string, NULL)
diff --git a/pandas/tests/test_algos.py b/pandas/tests/test_algos.py
index e0e4beffe113a..82f647c9385b2 100644
--- a/pandas/tests/test_algos.py
+++ b/pandas/tests/test_algos.py
@@ -1402,6 +1402,19 @@ class TestGroupVarFloat32(GroupVarTestMixin):
 
 
 class TestHashTable:
+    def test_string_hashtable_set_item_signature(self):
+        # GH#30419 fix typing in StringHashTable.set_item to prevent segfault
+        tbl = ht.StringHashTable()
+
+        tbl.set_item("key", 1)
+        assert tbl.get_item("key") == 1
+
+        with pytest.raises(TypeError, match="'key' has incorrect type"):
+            # key arg typed as string, not object
+            tbl.set_item(4, 6)
+        with pytest.raises(TypeError, match="'val' has incorrect type"):
+            tbl.get_item(4)
+
     def test_lookup_nan(self, writable):
         xs = np.array([2.718, 3.14, np.nan, -7, 5, 2, 3])
         # GH 21688 ensure we can deal with readonly memory views