diff --git a/icu4c/source/common/ucurr.cpp b/icu4c/source/common/ucurr.cpp
index 885ca3a922..f2cae13ce3 100644
--- a/icu4c/source/common/ucurr.cpp
+++ b/icu4c/source/common/ucurr.cpp
@@ -25,6 +25,7 @@
 #include "uenumimp.h"
 #include "uhash.h"
 #include "hash.h"
+#include "uinvchar.h"
 #include "uresimp.h"
 #include "ulist.h"
 #include "ureslocs.h"
@@ -545,93 +546,97 @@ U_CAPI int32_t U_EXPORT2
 ucurr_forLocale(const char* locale,
                 UChar* buff,
                 int32_t buffCapacity,
-                UErrorCode* ec)
-{
-    int32_t resLen = 0;
-    const UChar* s = NULL;
-    if (ec != NULL && U_SUCCESS(*ec)) {
-        if ((buff && buffCapacity) || !buffCapacity) {
-            UErrorCode localStatus = U_ZERO_ERROR;
-            char id[ULOC_FULLNAME_CAPACITY];
-            if ((resLen = uloc_getKeywordValue(locale, "currency", id, ULOC_FULLNAME_CAPACITY, &localStatus))) {
-                // there is a currency keyword. Try to see if it's valid
-                if(buffCapacity > resLen) {
-                    /* Normalize the currency keyword value to upper case. */
-                    T_CString_toUpperCase(id);
-                    u_charsToUChars(id, buff, resLen);
-                }
-            } else {
-                // get country or country_variant in `id'
-                uint32_t variantType = idForLocale(locale, id, sizeof(id), ec);
+                UErrorCode* ec) {
+    if (U_FAILURE(*ec)) { return 0; }
+    if (buffCapacity < 0 || (buff == nullptr && buffCapacity > 0)) {
+        *ec = U_ILLEGAL_ARGUMENT_ERROR;
+        return 0;
+    }
 
-                if (U_FAILURE(*ec)) {
-                    return 0;
-                }
+    char currency[4];  // ISO currency codes are alpha3 codes.
+    UErrorCode localStatus = U_ZERO_ERROR;
+    int32_t resLen = uloc_getKeywordValue(locale, "currency",
+                                          currency, UPRV_LENGTHOF(currency), &localStatus);
+    if (U_SUCCESS(localStatus) && resLen == 3 && uprv_isInvariantString(currency, resLen)) {
+        if (resLen < buffCapacity) {
+            T_CString_toUpperCase(currency);
+            u_charsToUChars(currency, buff, resLen);
+        }
+        return u_terminateUChars(buff, buffCapacity, resLen, ec);
+    }
+
+    // get country or country_variant in `id'
+    char id[ULOC_FULLNAME_CAPACITY];
+    uint32_t variantType = idForLocale(locale, id, UPRV_LENGTHOF(id), ec);
+    if (U_FAILURE(*ec)) {
+        return 0;
+    }
 
 #if !UCONFIG_NO_SERVICE
-                const UChar* result = CReg::get(id);
-                if (result) {
-                    if(buffCapacity > u_strlen(result)) {
-                        u_strcpy(buff, result);
-                    }
-                    return u_strlen(result);
-                }
+    const UChar* result = CReg::get(id);
+    if (result) {
+        if(buffCapacity > u_strlen(result)) {
+            u_strcpy(buff, result);
+        }
+        resLen = u_strlen(result);
+        return u_terminateUChars(buff, buffCapacity, resLen, ec);
+    }
 #endif
-                // Remove variants, which is only needed for registration.
-                char *idDelim = strchr(id, VAR_DELIM);
-                if (idDelim) {
-                    idDelim[0] = 0;
-                }
+    // Remove variants, which is only needed for registration.
+    char *idDelim = uprv_strchr(id, VAR_DELIM);
+    if (idDelim) {
+        idDelim[0] = 0;
+    }
 
-                // Look up the CurrencyMap element in the root bundle.
-                UResourceBundle *rb = ures_openDirect(U_ICUDATA_CURR, CURRENCY_DATA, &localStatus);
-                UResourceBundle *cm = ures_getByKey(rb, CURRENCY_MAP, rb, &localStatus);
-                UResourceBundle *countryArray = ures_getByKey(rb, id, cm, &localStatus);
-                UResourceBundle *currencyReq = ures_getByIndex(countryArray, 0, NULL, &localStatus);
+    const UChar* s;  // Currency code from data file.
+    if (id[0] == 0) {
+        // No point looking in the data for an empty string.
+        // This is what we would get.
+        localStatus = U_MISSING_RESOURCE_ERROR;
+    } else {
+        // Look up the CurrencyMap element in the root bundle.
+        localStatus = U_ZERO_ERROR;
+        UResourceBundle *rb = ures_openDirect(U_ICUDATA_CURR, CURRENCY_DATA, &localStatus);
+        UResourceBundle *cm = ures_getByKey(rb, CURRENCY_MAP, rb, &localStatus);
+        UResourceBundle *countryArray = ures_getByKey(rb, id, cm, &localStatus);
+        UResourceBundle *currencyReq = ures_getByIndex(countryArray, 0, NULL, &localStatus);
+        s = ures_getStringByKey(currencyReq, "id", &resLen, &localStatus);
+
+        // Get the second item when PREEURO is requested, and this is a known Euro country.
+        // If the requested variant is PREEURO, and this isn't a Euro country,
+        // assume that the country changed over to the Euro in the future.
+        // This is probably an old version of ICU that hasn't been updated yet.
+        // The latest currency is probably correct.
+        if (U_SUCCESS(localStatus)) {
+            if ((variantType & VARIANT_IS_PREEURO) && u_strcmp(s, EUR_STR) == 0) {
+                currencyReq = ures_getByIndex(countryArray, 1, currencyReq, &localStatus);
                 s = ures_getStringByKey(currencyReq, "id", &resLen, &localStatus);
-
-                /*
-                Get the second item when PREEURO is requested, and this is a known Euro country.
-                If the requested variant is PREEURO, and this isn't a Euro country, assume
-                that the country changed over to the Euro in the future. This is probably
-                an old version of ICU that hasn't been updated yet. The latest currency is
-                probably correct.
-                */
-                if (U_SUCCESS(localStatus)) {
-                    if ((variantType & VARIANT_IS_PREEURO) && u_strcmp(s, EUR_STR) == 0) {
-                        currencyReq = ures_getByIndex(countryArray, 1, currencyReq, &localStatus);
-                        s = ures_getStringByKey(currencyReq, "id", &resLen, &localStatus);
-                    }
-                    else if ((variantType & VARIANT_IS_EURO)) {
-                        s = EUR_STR;
-                    }
-                }
-                ures_close(countryArray);
-                ures_close(currencyReq);
-
-                if ((U_FAILURE(localStatus)) && strchr(id, '_') != 0)
-                {
-                    // We don't know about it.  Check to see if we support the variant.
-                    uloc_getParent(locale, id, sizeof(id), ec);
-                    *ec = U_USING_FALLBACK_WARNING;
-                    return ucurr_forLocale(id, buff, buffCapacity, ec);
-                }
-                else if (*ec == U_ZERO_ERROR || localStatus != U_ZERO_ERROR) {
-                    // There is nothing to fallback to. Report the failure/warning if possible.
-                    *ec = localStatus;
-                }
-                if (U_SUCCESS(*ec)) {
-                    if(buffCapacity > resLen) {
-                        u_strcpy(buff, s);
-                    }
-                }
+            } else if ((variantType & VARIANT_IS_EURO)) {
+                s = EUR_STR;
             }
-            return u_terminateUChars(buff, buffCapacity, resLen, ec);
-        } else {
-            *ec = U_ILLEGAL_ARGUMENT_ERROR;
+        }
+        ures_close(currencyReq);
+        ures_close(countryArray);
+    }
+
+    if ((U_FAILURE(localStatus)) && strchr(id, '_') != 0) {
+        // We don't know about it.  Check to see if we support the variant.
+        uloc_getParent(locale, id, UPRV_LENGTHOF(id), ec);
+        *ec = U_USING_FALLBACK_WARNING;
+        // TODO: Loop over the shortened id rather than recursing and
+        // looking again for a currency keyword.
+        return ucurr_forLocale(id, buff, buffCapacity, ec);
+    }
+    if (*ec == U_ZERO_ERROR || localStatus != U_ZERO_ERROR) {
+        // There is nothing to fallback to. Report the failure/warning if possible.
+        *ec = localStatus;
+    }
+    if (U_SUCCESS(*ec)) {
+        if(buffCapacity > resLen) {
+            u_strcpy(buff, s);
         }
     }
-    return resLen;
+    return u_terminateUChars(buff, buffCapacity, resLen, ec);
 }
 
 // end registration
diff --git a/icu4c/source/i18n/dcfmtsym.cpp b/icu4c/source/i18n/dcfmtsym.cpp
index 08a85b545e..43eea49ea5 100644
--- a/icu4c/source/i18n/dcfmtsym.cpp
+++ b/icu4c/source/i18n/dcfmtsym.cpp
@@ -433,12 +433,13 @@ DecimalFormatSymbols::initialize(const Locale& loc, UErrorCode& status, UBool us
     UErrorCode internalStatus = U_ZERO_ERROR; // don't propagate failures out
     UChar curriso[4];
     UnicodeString tempStr;
-    ucurr_forLocale(locStr, curriso, 4, &internalStatus);
-
-    uprv_getStaticCurrencyName(curriso, locStr, tempStr, internalStatus);
-    if (U_SUCCESS(internalStatus)) {
-        fSymbols[kIntlCurrencySymbol].setTo(curriso, -1);
-        fSymbols[kCurrencySymbol] = tempStr;
+    int32_t currisoLength = ucurr_forLocale(locStr, curriso, UPRV_LENGTHOF(curriso), &internalStatus);
+    if (U_SUCCESS(internalStatus) && currisoLength == 3) {
+        uprv_getStaticCurrencyName(curriso, locStr, tempStr, internalStatus);
+        if (U_SUCCESS(internalStatus)) {
+            fSymbols[kIntlCurrencySymbol].setTo(curriso, currisoLength);
+            fSymbols[kCurrencySymbol] = tempStr;
+        }
     }
     /* else use the default values. */
 
diff --git a/icu4c/source/test/intltest/loctest.cpp b/icu4c/source/test/intltest/loctest.cpp
index e849b26fa1..5bb2dc24dd 100644
--- a/icu4c/source/test/intltest/loctest.cpp
+++ b/icu4c/source/test/intltest/loctest.cpp
@@ -1257,6 +1257,19 @@ LocaleTest::TestEuroSupport()
     if (invalidLen || U_SUCCESS(status)) {
         errln("Fail: en_QQ didn't return NULL");
     }
+
+    // The currency keyword value is as long as the destination buffer.
+    // It should detect the overflow internally, and default to the locale's currency.
+    tmp[0] = u'¤';
+    status = U_ZERO_ERROR;
+    int32_t length = ucurr_forLocale("en_US@currency=euro", tmp, 4, &status);
+    if (U_FAILURE(status) || dollarStr != UnicodeString(tmp, length)) {
+        if (U_SUCCESS(status) && tmp[0] == u'¤') {
+            errln("Fail: ucurr_forLocale(en_US@currency=euro) succeeded without writing output");
+        } else {
+            errln("Fail: ucurr_forLocale(en_US@currency=euro) != USD - %s", u_errorName(status));
+        }
+    }
 }
 
 #endif