diff --git a/icu4c/source/common/uloc.cpp b/icu4c/source/common/uloc.cpp
index 73b4320..e032229 100644
--- a/icu4c/source/common/uloc.cpp
+++ b/icu4c/source/common/uloc.cpp
@@ -148,7 +148,8 @@
     "mad", "maf", "mag", "mai", "mak", "man", "mas", "mde",
     "mdf", "mdh", "mdr", "men", "mer", "mfe", "mg",  "mga",
     "mgh", "mgo", "mh",  "mi",  "mic", "min", "mis", "mk",
-    "ml",  "mn",  "mnc", "mni", "moh", "mos", "mr",  "mrj",
+    "ml",  "mn",  "mnc", "mni", "mo",
+    "moh", "mos", "mr",  "mrj",
     "ms",  "mt",  "mua", "mul", "mus", "mwl", "mwr", "mwv",
     "my",  "mye", "myv", "mzn",
     "na",  "nan", "nap", "naq", "nb",  "nd",  "nds", "ne",
@@ -264,7 +265,8 @@
     "mad", "maf", "mag", "mai", "mak", "man", "mas", "mde",
     "mdf", "mdh", "mdr", "men", "mer", "mfe", "mlg", "mga",
     "mgh", "mgo", "mah", "mri", "mic", "min", "mis", "mkd",
-    "mal", "mon", "mnc", "mni", "moh", "mos", "mar", "mrj",
+    "mal", "mon", "mnc", "mni", "mol",
+    "moh", "mos", "mar", "mrj",
     "msa", "mlt", "mua", "mul", "mus", "mwl", "mwr", "mwv",
     "mya", "mye", "myv", "mzn",
     "nau", "nan", "nap", "naq", "nob", "nde", "nds", "nep",
diff --git a/icu4c/source/test/intltest/loctest.cpp b/icu4c/source/test/intltest/loctest.cpp
index 79cb887..356ef64 100644
--- a/icu4c/source/test/intltest/loctest.cpp
+++ b/icu4c/source/test/intltest/loctest.cpp
@@ -221,6 +221,7 @@
     TESTCASE_AUTO(Test4147317);
     TESTCASE_AUTO(Test4147552);
     TESTCASE_AUTO(TestVariantParsing);
+    TESTCASE_AUTO(Test20639_DeprecatesISO3Language);
 #if !UCONFIG_NO_FORMATTING
     TESTCASE_AUTO(Test4105828);
 #endif
@@ -901,8 +902,8 @@
       ;
 
     /* TODO: Change this test to be more like the cloctst version? */
-    if (testCount != 595)
-        errln("Expected getISOLanguages() to return 595 languages; it returned %d", testCount);
+    if (testCount != 596)
+        errln("Expected getISOLanguages() to return 596 languages; it returned %d", testCount);
     else {
         for (i = 0; i < 15; i++) {
             int32_t j;
@@ -1577,6 +1578,27 @@
     }
 }
 
+void LocaleTest::Test20639_DeprecatesISO3Language() {
+    IcuTestErrorCode status(*this, "Test20639_DeprecatesISO3Language");
+
+    const struct TestCase {
+        const char* localeName;
+        const char* expectedISO3Language;
+    } cases[] = {
+        {"nb", "nob"},
+        {"no", "nor"}, // why not nob?
+        {"he", "heb"},
+        {"iw", "heb"},
+        {"ro", "ron"},
+        {"mo", "mol"},
+    };
+    for (auto& cas : cases) {
+        Locale loc(cas.localeName);
+        const char* actual = loc.getISO3Language();
+        assertEquals(cas.localeName, cas.expectedISO3Language, actual);
+    }
+}
+
 #if !UCONFIG_NO_FORMATTING
 
 /**
diff --git a/icu4c/source/test/intltest/loctest.h b/icu4c/source/test/intltest/loctest.h
index 6b6b3a6..72e3a82 100644
--- a/icu4c/source/test/intltest/loctest.h
+++ b/icu4c/source/test/intltest/loctest.h
@@ -69,6 +69,8 @@
     void Test4147315(void);
     void Test4147317(void);
     void Test4147552(void);
+
+    void Test20639_DeprecatesISO3Language();
     
     void TestVariantParsing(void);
 
diff --git a/icu4c/source/test/testdata/structLocale.txt b/icu4c/source/test/testdata/structLocale.txt
index 466ba9d..c30a534 100644
--- a/icu4c/source/test/testdata/structLocale.txt
+++ b/icu4c/source/test/testdata/structLocale.txt
@@ -5806,6 +5806,7 @@
         mn{""}
         mnc{""}
         mni{""}
+        mo{""}
         moh{""}
         mos{""}
         mr{""}
diff --git a/icu4j/main/tests/localespi/src/com/ibm/icu/dev/test/localespi/LocaleNameTest.java b/icu4j/main/tests/localespi/src/com/ibm/icu/dev/test/localespi/LocaleNameTest.java
index 651f436..c13bc0c 100644
--- a/icu4j/main/tests/localespi/src/com/ibm/icu/dev/test/localespi/LocaleNameTest.java
+++ b/icu4j/main/tests/localespi/src/com/ibm/icu/dev/test/localespi/LocaleNameTest.java
@@ -276,4 +276,21 @@
             }
         }
     }
+
+    @Test
+    public void Test20639_DeprecatesISO3Language() {
+        String[][] cases = new String[][]{
+            {"nb", "nob"},
+            {"no", "nor"}, // why not nob?
+            {"he", "heb"},
+            {"iw", "heb"},
+            {"ro", "ron"},
+            {"mo", "mol"},
+        };
+        for (String[] cas : cases) {
+            ULocale loc = new ULocale(cas[0]);
+            String actual = loc.getISO3Language();
+            assertEquals(cas[0], cas[1], actual);
+        }
+    }
 }