diff --git a/src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs b/src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs
index 35b7b7973ffcc7..080a24d625b106 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs
@@ -3744,24 +3744,36 @@ private static bool MatchTimeMark(ref __DTString str, DateTimeFormatInfo dtfi, s
 
             if (str.GetNext())
             {
-                string searchStr = dtfi.AMDesignator;
-                if (searchStr.Length > 0)
+                string amDesignator = dtfi.AMDesignator;
+                string pmDesignator;
+
+                if (amDesignator.Length > 0)
                 {
-                    if (str.MatchSpecifiedWord(searchStr))
+                    if (str.MatchSpecifiedWord(amDesignator))
                     {
+                        pmDesignator = dtfi.PMDesignator;
+                        if (pmDesignator.StartsWith(amDesignator, StringComparison.Ordinal) && str.MatchSpecifiedWord(pmDesignator))
+                        {
+                            // AM designator is a prefix of PM designator and we have matched PM designator. Use longer match.
+                            str.Index += (pmDesignator.Length - 1);
+                            result = TM.PM;
+                            return true;
+                        }
+
                         // Found an AM timemark with length > 0.
-                        str.Index += (searchStr.Length - 1);
+                        str.Index += (amDesignator.Length - 1);
                         result = TM.AM;
                         return true;
                     }
                 }
-                searchStr = dtfi.PMDesignator;
-                if (searchStr.Length > 0)
+
+                pmDesignator = dtfi.PMDesignator;
+                if (pmDesignator.Length > 0)
                 {
-                    if (str.MatchSpecifiedWord(searchStr))
+                    if (str.MatchSpecifiedWord(pmDesignator))
                     {
                         // Found a PM timemark with length > 0.
-                        str.Index += (searchStr.Length - 1);
+                        str.Index += (pmDesignator.Length - 1);
                         result = TM.PM;
                         return true;
                     }
diff --git a/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/DateTimeTests.cs b/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/DateTimeTests.cs
index 716b4df993a6d3..1e310009f52fdb 100644
--- a/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/DateTimeTests.cs
+++ b/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/DateTimeTests.cs
@@ -3085,5 +3085,22 @@ public void TestDateTimeCalculationPrecision(double value, DateTime initialValue
                 break;
             }
         }
+
+        [Fact]
+        public void TestParsingWithAmPrefixPm()
+        {
+            var culture = new CultureInfo("en-US");
+            DateTime dt = new DateTime(2023, 4, 17, 14, 30, 0);
+
+            // AM designator is a prefix of PM designator
+            culture.DateTimeFormat.AMDesignator = "aM";
+            culture.DateTimeFormat.PMDesignator = "aMP";
+
+            string formatted = dt.ToString("hh:mm tt", culture);
+            Assert.Equal("02:30 aMP", formatted);
+
+            DateTime parsedDateTime = DateTime.ParseExact(formatted, "hh:mm tt", culture);
+            Assert.Equal(dt.TimeOfDay, parsedDateTime.TimeOfDay);
+        }
     }
 }