diff --git a/prometheus_client/samples.py b/prometheus_client/samples.py
index 994d1281..6eac7d64 100644
--- a/prometheus_client/samples.py
+++ b/prometheus_client/samples.py
@@ -13,7 +13,8 @@ def __init__(self, sec: float, nsec: float) -> None:
         self.nsec: int = int(nsec)
 
     def __str__(self) -> str:
-        return f"{self.sec}.{self.nsec:09d}"
+        sign = '-' if self.sec < 0 or self.nsec < 0 else ''
+        return f"{sign}{abs(self.sec)}.{abs(self.nsec):09d}"
 
     def __repr__(self) -> str:
         return f"Timestamp({self.sec}, {self.nsec})"
diff --git a/tests/openmetrics/test_exposition.py b/tests/openmetrics/test_exposition.py
index 3ba4031a..5abc0423 100644
--- a/tests/openmetrics/test_exposition.py
+++ b/tests/openmetrics/test_exposition.py
@@ -492,6 +492,8 @@ def collect(self):
                 metric.add_sample("ts", {"foo": "d"}, 0, Timestamp(123, 456000000))
                 metric.add_sample("ts", {"foo": "e"}, 0, Timestamp(123, 456000))
                 metric.add_sample("ts", {"foo": "f"}, 0, Timestamp(123, 456))
+                metric.add_sample("ts", {"foo": "g"}, 0, Timestamp(-123, 456000000))
+                metric.add_sample("ts", {"foo": "h"}, 0, Timestamp(-123, 456))
                 yield metric
 
         self.registry.register(MyCollector())
@@ -503,6 +505,8 @@ def collect(self):
 ts{foo="d"} 0.0 123.456000000
 ts{foo="e"} 0.0 123.000456000
 ts{foo="f"} 0.0 123.000000456
+ts{foo="g"} 0.0 -123.456000000
+ts{foo="h"} 0.0 -123.000000456
 # EOF
 """, generate_latest(self.registry, version="1.0.0"))
 
diff --git a/tests/test_samples.py b/tests/test_samples.py
index 7b59218b..e1e416da 100644
--- a/tests/test_samples.py
+++ b/tests/test_samples.py
@@ -4,6 +4,23 @@
 
 
 class TestSamples(unittest.TestCase):
+    def test_negative_timestamp_string(self):
+        for nsec, expected in [
+            (0, '-123.000000000'),
+            (1, '-123.000000001'),
+            (456000000, '-123.456000000'),
+            (999999999, '-123.999999999'),
+        ]:
+            with self.subTest(nsec=nsec):
+                timestamp = samples.Timestamp(-123, nsec)
+                self.assertEqual(str(timestamp), expected)
+                self.assertEqual(float(str(timestamp)), float(timestamp))
+
+    def test_negative_timestamp_string_with_zero_seconds(self):
+        timestamp = samples.Timestamp(-0.5, 500000000)
+        self.assertEqual(str(timestamp), '-0.500000000')
+        self.assertEqual(float(str(timestamp)), float(timestamp))
+
     def test_gt(self):
         self.assertEqual(samples.Timestamp(1, 1) > samples.Timestamp(1, 1), False)
         self.assertEqual(samples.Timestamp(1, 1) > samples.Timestamp(1, 2), False)