diff --git a/changelog.md b/changelog.md
index b3ea3a70..5dae202e 100644
--- a/changelog.md
+++ b/changelog.md
@@ -8,6 +8,7 @@
 - fix Unix timezone fallback offset sign and DST abbreviation
 - fix JavaScript getTimeZone offsets and summer flag around DST transitions
 - fix numeric parsers accepting overflowing bounded values
+- fix systemToPOSIXTime for SystemTime leap-second values
 
 ## [1.16] - 2026-05-03
 
diff --git a/lib/Data/Time/Clock/POSIX.hs b/lib/Data/Time/Clock/POSIX.hs
index 8e499d64..135aa322 100644
--- a/lib/Data/Time/Clock/POSIX.hs
+++ b/lib/Data/Time/Clock/POSIX.hs
@@ -45,7 +45,12 @@ utcTimeToPOSIXSeconds (UTCTime d t) =
     (fromInteger (diffDays d systemEpochDay) * posixDayLength) + min posixDayLength (realToFrac t)
 
 systemToPOSIXTime :: SystemTime -> POSIXTime
-systemToPOSIXTime (MkSystemTime s ns) = (fromIntegral s) + (fromIntegral ns) * 1E-9
+systemToPOSIXTime (MkSystemTime s ns) =
+    let
+        (days, timeSeconds) = s `divMod` 86400
+        time = fromIntegral timeSeconds + fromIntegral ns * 1E-9
+    in
+        fromIntegral days * posixDayLength + min posixDayLength time
 
 -- | Get the current POSIX time from the system clock.
 getPOSIXTime :: IO POSIXTime
diff --git a/test/main/Test/Clock/Conversion.hs b/test/main/Test/Clock/Conversion.hs
index 020ee0d9..b5c04a22 100644
--- a/test/main/Test/Clock/Conversion.hs
+++ b/test/main/Test/Clock/Conversion.hs
@@ -3,6 +3,7 @@ module Test.Clock.Conversion (
 ) where
 
 import Data.Time.Clock
+import Data.Time.Clock.POSIX
 import Data.Time.Clock.System
 import Test.Tasty
 import Test.Tasty.HUnit
@@ -17,6 +18,18 @@ testClockConversion =
                     [ testCase "systemToUTCTime" $ assertEqual (show ut) ut $ systemToUTCTime st
                     , testCase "utcToSystemTime" $ assertEqual (show ut) st $ utcToSystemTime ut
                     ]
+            testPOSIXTime :: (SystemTime, POSIXTime) -> TestTree
+            testPOSIXTime (st, pt) =
+                testGroup
+                    (show st)
+                    [ testCase "expected" $
+                        assertEqual (show st) pt $ systemToPOSIXTime st
+                    , testCase "matches systemToUTCTime" $
+                        assertEqual
+                            (show st)
+                            (utcTimeToPOSIXSeconds $ systemToUTCTime st)
+                            (systemToPOSIXTime st)
+                    ]
         in
             [ testPair (MkSystemTime 0 0, UTCTime systemEpochDay 0)
             , testPair (MkSystemTime 86399 0, UTCTime systemEpochDay 86399)
@@ -24,4 +37,13 @@ testClockConversion =
             , testPair (MkSystemTime 86399 1000000000, UTCTime systemEpochDay 86400)
             , testPair (MkSystemTime 86399 1999999999, UTCTime systemEpochDay 86400.999999999)
             , testPair (MkSystemTime 86400 0, UTCTime (succ systemEpochDay) 0)
+            , testGroup
+                "systemToPOSIXTime"
+                [ testPOSIXTime (MkSystemTime 0 0, 0)
+                , testPOSIXTime (MkSystemTime 86399 999999999, 86399.999999999)
+                , testPOSIXTime (MkSystemTime 86399 1000000000, 86400)
+                , testPOSIXTime (MkSystemTime 86399 1500000000, 86400)
+                , testPOSIXTime (MkSystemTime 86399 1999999999, 86400)
+                , testPOSIXTime (MkSystemTime 86400 0, 86400)
+                ]
             ]