diff --git a/changelog/6820.improvement.rst b/changelog/6820.improvement.rst
new file mode 100644
index 00000000000..75f709a88ea
--- /dev/null
+++ b/changelog/6820.improvement.rst
@@ -0,0 +1 @@
+Add support for asserting complete output with :class:`~pytest.LineMatcher`'s :func:`~pytest.LineMatcher.fnmatch_lines` and :func:`~pytest.LineMatcher.re_match_lines` via a new ``complete`` option.
diff --git a/src/_pytest/pytester.py b/src/_pytest/pytester.py
index bbc740e9582..07798c01752 100644
--- a/src/_pytest/pytester.py
+++ b/src/_pytest/pytester.py
@@ -1653,7 +1653,11 @@ def _log_text(self) -> str:
         return "\n".join(self._log_output)
 
     def fnmatch_lines(
-        self, lines2: Sequence[str], *, consecutive: bool = False
+        self,
+        lines2: Sequence[str],
+        *,
+        consecutive: bool = False,
+        complete: bool = False,
     ) -> None:
         """Check lines exist in the output (using :func:`python:fnmatch.fnmatch`).
 
@@ -1663,12 +1667,19 @@ def fnmatch_lines(
 
         :param lines2: String patterns to match.
         :param consecutive: Match lines consecutively?
+        :param complete: Fail if the output contains lines which were not asserted?
         """
         __tracebackhide__ = True
-        self._match_lines(lines2, fnmatch, "fnmatch", consecutive=consecutive)
+        self._match_lines(
+            lines2, fnmatch, "fnmatch", consecutive=consecutive, complete=complete
+        )
 
     def re_match_lines(
-        self, lines2: Sequence[str], *, consecutive: bool = False
+        self,
+        lines2: Sequence[str],
+        *,
+        consecutive: bool = False,
+        complete: bool = False,
     ) -> None:
         """Check lines exist in the output (using :func:`python:re.match`).
 
@@ -1679,6 +1690,7 @@ def re_match_lines(
 
         :param lines2: string patterns to match.
         :param consecutive: match lines consecutively?
+        :param complete: Fail if the output contains lines which were not asserted?
         """
         __tracebackhide__ = True
         self._match_lines(
@@ -1686,6 +1698,7 @@ def re_match_lines(
             lambda name, pat: bool(re.match(pat, name)),
             "re.match",
             consecutive=consecutive,
+            complete=complete,
         )
 
     def _match_lines(
@@ -1695,6 +1708,7 @@ def _match_lines(
         match_nickname: str,
         *,
         consecutive: bool = False,
+        complete: bool = False,
     ) -> None:
         """Underlying implementation of ``fnmatch_lines`` and ``re_match_lines``.
 
@@ -1710,6 +1724,8 @@ def _match_lines(
             when a match occurs.
         :param consecutive:
             Match lines consecutively?
+        :param complete:
+            Fail if the output contains lines which were not asserted?
         """
         if not isinstance(lines2, collections.abc.Sequence):
             raise TypeError(f"invalid type for lines2: {type(lines2).__name__}")
@@ -1753,6 +1769,12 @@ def _match_lines(
                 msg = f"remains unmatched: {line!r}"
                 self._log(msg)
                 self._fail(msg)
+        if complete:
+            unasserted = extralines + lines1
+            if unasserted:
+                msg = f"output has unasserted lines: {unasserted!r}"
+                self._log(msg)
+                self._fail(msg)
         self._log_output = []
 
     def no_fnmatch_line(self, pat: str) -> None:
diff --git a/testing/test_pytester.py b/testing/test_pytester.py
index e9a20f1063c..fb133f2ca38 100644
--- a/testing/test_pytester.py
+++ b/testing/test_pytester.py
@@ -549,6 +549,61 @@ def test_linematcher_consecutive() -> None:
     ]
 
 
+def test_linematcher_complete() -> None:
+    lm = LineMatcher(["1", "2", "3"])
+    lm.fnmatch_lines(["1", "2", "3"], complete=True)
+    lm.re_match_lines(["1", "2", "3"], complete=True)
+
+    lm = LineMatcher(["unexpected", "1", "2", "3"])
+    with pytest.raises(pytest.fail.Exception) as excinfo:
+        lm.fnmatch_lines(["1", "2", "3"], complete=True)
+    assert str(excinfo.value).splitlines() == [
+        "nomatch: '1'",
+        "    and: 'unexpected'",
+        "exact match: '1'",
+        "exact match: '2'",
+        "exact match: '3'",
+        "output has unasserted lines: ['unexpected']",
+    ]
+
+    lm = LineMatcher(["1", "2", "3", "trailing"])
+    with pytest.raises(pytest.fail.Exception) as excinfo:
+        lm.re_match_lines(["1", "2", "3"], complete=True)
+    assert str(excinfo.value).splitlines() == [
+        "exact match: '1'",
+        "exact match: '2'",
+        "exact match: '3'",
+        "output has unasserted lines: ['trailing']",
+    ]
+
+    lm = LineMatcher(["0", "1", "2", "3"])
+    with pytest.raises(pytest.fail.Exception) as excinfo:
+        lm.fnmatch_lines(["1", "2", "3"], consecutive=True, complete=True)
+    assert str(excinfo.value).splitlines() == [
+        "nomatch: '1'",
+        "    and: '0'",
+        "exact match: '1'",
+        "exact match: '2'",
+        "exact match: '3'",
+        "output has unasserted lines: ['0']",
+    ]
+
+    lm = LineMatcher(["x"])
+    with pytest.raises(pytest.fail.Exception) as excinfo:
+        lm.fnmatch_lines([], complete=True)
+    assert str(excinfo.value).splitlines() == [
+        "output has unasserted lines: ['x']",
+    ]
+
+    lm = LineMatcher(["1", "1"])
+    with pytest.raises(pytest.fail.Exception) as excinfo:
+        lm.fnmatch_lines(["1"], complete=True)
+    assert str(excinfo.value).splitlines() == [
+        "exact match: '1'",
+        "output has unasserted lines: ['1']",
+    ]
+
+
 @pytest.mark.parametrize("function", ["no_fnmatch_line", "no_re_match_line"])
 def test_linematcher_no_matching(function: str) -> None:
     if function == "no_fnmatch_line":