diff --git a/Lib/test/test_tokenize.py b/Lib/test/test_tokenize.py
index 47f2c06685bcaa..63c2501cfe2338 100644
--- a/Lib/test/test_tokenize.py
+++ b/Lib/test/test_tokenize.py
@@ -10,6 +10,8 @@
 from unittest import TestCase, mock
 from test.test_grammar import (VALID_UNDERSCORE_LITERALS,
                                INVALID_UNDERSCORE_LITERALS)
+from test.support import os_helper
+from test.support.script_helper import run_test_script, make_script
 import os
 import token
 
@@ -2631,5 +2633,19 @@ def fib(n):
         self.assertEqual(get_tokens(code), get_tokens(code_no_cont))
 
 
+class CTokenizerBufferTests(unittest.TestCase):
+    def test_newline_at_the_end_of_buffer(self):
+        # See issue 99581: Make sure that if we need to add a new line at the
+        # end of the buffer, we have enough space in the buffer, specially when
+        # the current line is as long as the buffer space available.
+        test_script = f"""\
+        #coding: latin-1
+        #{"a"*10000}
+        #{"a"*10002}"""
+        with os_helper.temp_dir() as temp_dir:
+            file_name = make_script(temp_dir, 'foo', test_script)
+            run_test_script(file_name)
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-11-19-22-27-52.gh-issue-99581.yKYPbf.rst b/Misc/NEWS.d/next/Core and Builtins/2022-11-19-22-27-52.gh-issue-99581.yKYPbf.rst
new file mode 100644
index 00000000000000..8071fd130dd685
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2022-11-19-22-27-52.gh-issue-99581.yKYPbf.rst	
@@ -0,0 +1,3 @@
+Fixed a bug that was causing a buffer overflow if the tokenizer copies a
+line missing the newline caracter from a file that is as long as the
+available tokenizer buffer. Patch by Pablo galindo
diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
index a5cfb659b430a4..eda38a09a995ae 100644
--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -396,7 +396,11 @@ tok_readline_recode(struct tok_state *tok) {
         error_ret(tok);
         goto error;
     }
-    if (!tok_reserve_buf(tok, buflen + 1)) {
+    // Make room for the null terminator *and* potentially
+    // an extra newline character that we may need to artificially
+    // add.
+    size_t buffer_size = buflen + 2;
+    if (!tok_reserve_buf(tok, buffer_size)) {
         goto error;
     }
     memcpy(tok->inp, buf, buflen);
@@ -983,6 +987,7 @@ tok_underflow_file(struct tok_state *tok) {
         return 0;
     }
     if (tok->inp[-1] != '\n') {
+        assert(tok->inp + 1 < tok->end);
         /* Last line does not end in \n, fake one */
         *tok->inp++ = '\n';
         *tok->inp = '\0';