diff --git a/icu4c/source/i18n/number_decimalquantity.cpp b/icu4c/source/i18n/number_decimalquantity.cpp
index e646c546458..cd6e7b14d28 100644
--- a/icu4c/source/i18n/number_decimalquantity.cpp
+++ b/icu4c/source/i18n/number_decimalquantity.cpp
@@ -1008,13 +1008,8 @@ void DecimalQuantity::shiftLeft(int32_t numDigits) {
     }
     if (usingBytes) {
         ensureCapacity(precision + numDigits);
-        int i = precision + numDigits - 1;
-        for (; i >= numDigits; i--) {
-            fBCD.bcdBytes.ptr[i] = fBCD.bcdBytes.ptr[i - numDigits];
-        }
-        for (; i >= 0; i--) {
-            fBCD.bcdBytes.ptr[i] = 0;
-        }
+        uprv_memmove(fBCD.bcdBytes.ptr + numDigits, fBCD.bcdBytes.ptr, precision);
+        uprv_memset(fBCD.bcdBytes.ptr, 0, numDigits);
     } else {
         fBCD.bcdLong <<= (numDigits * 4);
     }
diff --git a/icu4c/source/test/intltest/numfmtst.cpp b/icu4c/source/test/intltest/numfmtst.cpp
index b1eb796bc1e..50a08c0b082 100644
--- a/icu4c/source/test/intltest/numfmtst.cpp
+++ b/icu4c/source/test/intltest/numfmtst.cpp
@@ -247,6 +247,7 @@ void NumberFormatTest::runIndexedTest( int32_t index, UBool exec, const char* &n
   TESTCASE_AUTO(Test20961_CurrencyPluralPattern);
   TESTCASE_AUTO(Test21134_ToNumberFormatter);
   TESTCASE_AUTO(Test13733_StrictAndLenient);
+  TESTCASE_AUTO(Test21232_ParseTimeout);
   TESTCASE_AUTO_END;
 }
 
@@ -10033,4 +10034,29 @@ void NumberFormatTest::Test13733_StrictAndLenient() {
     }
 }
 
+void NumberFormatTest::Test21232_ParseTimeout() {
+    IcuTestErrorCode status(*this, "Test21232_ParseTimeout");
+
+    DecimalFormat df(status);
+    if (status.errDataIfFailureAndReset()) {
+        return;
+    }
+
+    UnicodeString input = u"4444444444444444444444444444444444444444";
+    if (quick) {
+        for (int32_t i = 0; i < 5; i++) {
+            input.append(input);
+        }
+        assertEquals("Somewhat long input of digits", 1280, input.length());
+    } else {
+        for (int32_t i = 0; i < 12; i++) {
+            input.append(input);
+        }
+        assertEquals("Very long input of digits", 163840, input.length());
+    }
+    Formattable result;
+    df.parse(input, result, status);
+    // Should not hang
+}
+
 #endif /* #if !UCONFIG_NO_FORMATTING */
diff --git a/icu4c/source/test/intltest/numfmtst.h b/icu4c/source/test/intltest/numfmtst.h
index 4e2d797b107..24ca15a07b9 100644
--- a/icu4c/source/test/intltest/numfmtst.h
+++ b/icu4c/source/test/intltest/numfmtst.h
@@ -303,6 +303,7 @@ class NumberFormatTest: public CalendarTimeZoneTest {
     void Test20961_CurrencyPluralPattern();
     void Test21134_ToNumberFormatter();
     void Test13733_StrictAndLenient();
+    void Test21232_ParseTimeout();
 
  private:
     UBool testFormattableAsUFormattable(const char *file, int line, Formattable &f);
diff --git a/icu4j/main/classes/core/src/com/ibm/icu/impl/number/DecimalQuantity_DualStorageBCD.java b/icu4j/main/classes/core/src/com/ibm/icu/impl/number/DecimalQuantity_DualStorageBCD.java
index 05612af295e..0c25de32550 100644
--- a/icu4j/main/classes/core/src/com/ibm/icu/impl/number/DecimalQuantity_DualStorageBCD.java
+++ b/icu4j/main/classes/core/src/com/ibm/icu/impl/number/DecimalQuantity_DualStorageBCD.java
@@ -4,6 +4,7 @@ package com.ibm.icu.impl.number;
 
 import java.math.BigDecimal;
 import java.math.BigInteger;
+import java.util.Arrays;
 
 /**
  * A DecimalQuantity with internal storage as a 64-bit BCD, with fallback to a byte array for numbers
@@ -123,13 +124,8 @@ public final class DecimalQuantity_DualStorageBCD extends DecimalQuantity_Abstra
         }
         if (usingBytes) {
             ensureCapacity(precision + numDigits);
-            int i = precision + numDigits - 1;
-            for (; i >= numDigits; i--) {
-                bcdBytes[i] = bcdBytes[i - numDigits];
-            }
-            for (; i >= 0; i--) {
-                bcdBytes[i] = 0;
-            }
+            System.arraycopy(bcdBytes, 0, bcdBytes, numDigits, precision);
+            Arrays.fill(bcdBytes, 0, numDigits, (byte) 0);
         } else {
             bcdLong <<= (numDigits * 4);
         }
diff --git a/icu4j/main/tests/core/src/com/ibm/icu/dev/test/format/NumberFormatTest.java b/icu4j/main/tests/core/src/com/ibm/icu/dev/test/format/NumberFormatTest.java
index babe1594910..98ce7178e79 100644
--- a/icu4j/main/tests/core/src/com/ibm/icu/dev/test/format/NumberFormatTest.java
+++ b/icu4j/main/tests/core/src/com/ibm/icu/dev/test/format/NumberFormatTest.java
@@ -6906,4 +6906,17 @@ public class NumberFormatTest extends TestFmwk {
                     parsedLenientValue, expectedLenientParse);
         }
     }
+
+    @Test
+    public void Test21232_ParseTimeout() throws ParseException {
+        DecimalFormat df = new DecimalFormat();
+        StringBuilder input = new StringBuilder();
+        input.append("4444444444444444444444444444444444444444");
+        for (int i = 0; i < 8; i++) {
+            input.append(input);
+        }
+        assertEquals("Long input of digits", 10240, input.length());
+        df.parse(input.toString());
+        // Should not hang
+    }
 }