diff --git a/Misc/NEWS.d/next/Library/2026-07-29-14-20-40.gh-issue-154855.vfF453.rst b/Misc/NEWS.d/next/Library/2026-07-29-14-20-40.gh-issue-154855.vfF453.rst
new file mode 100644
index 000000000000000..d8895e32bafe8a9
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-29-14-20-40.gh-issue-154855.vfF453.rst
@@ -0,0 +1,3 @@
+Fix :meth:`curses.window.instr` returning one character too few when the
+:mod:`curses` module is built against a curses library that counts the
+terminator in the requested length, such as the NetBSD one.
diff --git a/Modules/_cursesmodule.c b/Modules/_cursesmodule.c
index 82c8e642a51154f..168bf78711cf5f9 100644
--- a/Modules/_cursesmodule.c
+++ b/Modules/_cursesmodule.c
@@ -310,6 +310,14 @@ curses_window_set_null_error(PyCursesWindowObject *win,
     _curses_set_null_error(state, curses_funcname, python_funcname);
 }
 
+/* ncurses and PDCurses store n characters and add a terminator; NetBSD
+   curses counts the terminator in n.  Ask an unknown library for one more. */
+#if defined(NCURSES_VERSION) || defined(PDCURSES)
+#  define CURSES_STR_EXTRA  0
+#else
+#  define CURSES_STR_EXTRA  1
+#endif
+
 /* Utility Checking Procedures */
 
 /*
@@ -2210,25 +2218,33 @@ PyCursesWindow_instr(PyObject *op, PyObject *args)
         return NULL;
     }
 
-    n = Py_MIN(n, max_buf_size - 1);
+    n = Py_MIN(n, max_buf_size - 1 - CURSES_STR_EXTRA);
+    n += CURSES_STR_EXTRA;
     PyBytesWriter *writer = PyBytesWriter_Create(n + 1);
     if (writer == NULL) {
         return NULL;
     }
     char *buf = PyBytesWriter_GetData(writer);
 
-    if (use_xy) {
-        rtn = mvwinnstr(self->win, y, x, buf, n);
-    }
-    else {
-        rtn = winnstr(self->win, buf, n);
+    /* Read again if the library stored more than asked: truncating could
+       split a multibyte character. */
+    for (unsigned int want = n - CURSES_STR_EXTRA; ; n = want) {
+        if (use_xy) {
+            rtn = mvwinnstr(self->win, y, x, buf, n);
+        }
+        else {
+            rtn = winnstr(self->win, buf, n);
+        }
+        if (rtn == ERR || (unsigned int)rtn <= want) {
+            break;
+        }
     }
 
     if (rtn == ERR) {
         PyBytesWriter_Discard(writer);
         return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
     }
-    return PyBytesWriter_FinishWithSize(writer, strlen(buf));
+    return PyBytesWriter_FinishWithSize(writer, rtn);
 }
 
 /*[clinic input]