diff --git a/icu4c/source/i18n/number_longnames.cpp b/icu4c/source/i18n/number_longnames.cpp
index 97a5973..8b33992 100644
--- a/icu4c/source/i18n/number_longnames.cpp
+++ b/icu4c/source/i18n/number_longnames.cpp
@@ -455,6 +455,8 @@
         status = U_UNSUPPORTED_ERROR;
         return &micros.helpers.emptyWeakModifier;
     }
+    // If we don't have at least one mixedMeasure, the LongNameHandler would be
+    // sufficient and we shouldn't be running MixedUnitLongNameHandler code:
     U_ASSERT(micros.mixedMeasuresCount > 0);
     // mixedMeasures does not contain the last value:
     U_ASSERT(fMixedUnitCount == micros.mixedMeasuresCount + 1);
@@ -486,6 +488,11 @@
     for (int32_t i = 0; i < micros.mixedMeasuresCount; i++) {
         DecimalQuantity fdec;
         fdec.setToLong(micros.mixedMeasures[i]);
+        if (i > 0 && fdec.isNegative()) {
+            // If numbers are negative, only the first number needs to have its
+            // negative sign formatted.
+            fdec.negate();
+        }
         StandardPlural::Form pluralForm = utils::getStandardPlural(rules, fdec);
 
         UnicodeString simpleFormat =
@@ -499,6 +506,13 @@
         // TODO(icu-units#67): fix field positions
     }
 
+    // Reiterated: we have at least one mixedMeasure:
+    U_ASSERT(micros.mixedMeasuresCount > 0);
+    // Thus if negative, a negative has already been formatted:
+    if (quantity.isNegative()) {
+        quantity.negate();
+    }
+
     UnicodeString *finalSimpleFormats = &fMixedUnitData[(fMixedUnitCount - 1) * ARRAY_LENGTH];
     StandardPlural::Form finalPlural = utils::getPluralSafe(micros.rounder, rules, quantity, status);
     UnicodeString finalSimpleFormat = getWithPlural(finalSimpleFormats, finalPlural, status);
diff --git a/icu4c/source/i18n/units_complexconverter.cpp b/icu4c/source/i18n/units_complexconverter.cpp
index 9775b6a..27f835e 100644
--- a/icu4c/source/i18n/units_complexconverter.cpp
+++ b/icu4c/source/i18n/units_complexconverter.cpp
@@ -110,9 +110,13 @@
 MaybeStackVector<Measure> ComplexUnitsConverter::convert(double quantity,
                                                          icu::number::impl::RoundingImpl *rounder,
                                                          UErrorCode &status) const {
-    // TODO(icu-units#63): test negative numbers!
     // TODO(hugovdm): return an error for "foot-and-foot"?
     MaybeStackVector<Measure> result;
+    int sign = 1;
+    if (quantity < 0) {
+        quantity *= -1;
+        sign = -1;
+    }
 
     // For N converters:
     // - the first converter converts from the input unit to the largest unit,
@@ -190,7 +194,7 @@
     // Package values into Measure instances in result:
     for (int i = 0, n = unitConverters_.length(); i < n; ++i) {
         if (i < n - 1) {
-            Formattable formattableQuantity(intValues[i]);
+            Formattable formattableQuantity(intValues[i] * sign);
             // Measure takes ownership of the MeasureUnit*
             MeasureUnit *type = new MeasureUnit(units_[i]->copy(status).build(status));
             if (result.emplaceBackAndCheckErrorCode(status, formattableQuantity, type, status) ==
@@ -204,7 +208,7 @@
             }
         } else { // LAST ELEMENT
             // Add the last element, not an integer:
-            Formattable formattableQuantity(quantity);
+            Formattable formattableQuantity(quantity * sign);
             // Measure takes ownership of the MeasureUnit*
             MeasureUnit *type = new MeasureUnit(units_[i]->copy(status).build(status));
             if (result.emplaceBackAndCheckErrorCode(status, formattableQuantity, type, status) ==
diff --git a/icu4c/source/i18n/units_router.cpp b/icu4c/source/i18n/units_router.cpp
index 9489a8d..3158718 100644
--- a/icu4c/source/i18n/units_router.cpp
+++ b/icu4c/source/i18n/units_router.cpp
@@ -15,6 +15,7 @@
 #include "unicode/measure.h"
 #include "units_data.h"
 #include "units_router.h"
+#include <cmath>
 
 U_NAMESPACE_BEGIN
 namespace units {
@@ -96,7 +97,7 @@
     const ConverterPreference *converterPreference = nullptr;
     for (int32_t i = 0, n = converterPreferences_.length(); i < n; i++) {
         converterPreference = converterPreferences_[i];
-        if (converterPreference->converter.greaterThanOrEqual(quantity * (1 + DBL_EPSILON),
+        if (converterPreference->converter.greaterThanOrEqual(std::abs(quantity) * (1 + DBL_EPSILON),
                                                               converterPreference->limit)) {
             break;
         }
diff --git a/icu4c/source/test/intltest/numbertest_api.cpp b/icu4c/source/test/intltest/numbertest_api.cpp
index ca2170d..43323fb 100644
--- a/icu4c/source/test/intltest/numbertest_api.cpp
+++ b/icu4c/source/test/intltest/numbertest_api.cpp
@@ -764,6 +764,24 @@
             Locale("en-US"),
             1.9999,
             u"2 feet, 0 inches");
+
+    assertFormatSingle(
+            u"Negative numbers: temperature",
+            u"measure-unit/temperature-celsius",
+            u"unit/celsius",
+            NumberFormatter::with().unit(MeasureUnit::forIdentifier("celsius", status)),
+            Locale("nl-NL"),
+            -6.5,
+            u"-6,5\u00B0C");
+
+    assertFormatSingle(
+            u"Negative numbers: time",
+            nullptr, // submitting after TODO(icu-units#35) is fixed: fill in skeleton!
+            u"unit/hour-and-minute-and-second",
+            NumberFormatter::with().unit(MeasureUnit::forIdentifier("hour-and-minute-and-second", status)),
+            Locale("de-DE"),
+            -1.24,
+            u"-1 Std., 14 Min. und 24 Sek.");
 }
 
 void NumberFormatterApiTest::unitCompoundMeasure() {
@@ -863,6 +881,16 @@
             2.4,
             "2.4 m/s/s");
 
+    assertFormatSingle(
+            u"Negative numbers: acceleration",
+            u"measure-unit/acceleration-meter-per-square-second",
+            // TODO: when other PRs are merged, try: u"unit/meter-per-second-second" instead:
+            u"measure-unit/acceleration-meter-per-square-second",
+            NumberFormatter::with().unit(MeasureUnit::forIdentifier("meter-per-pow2-second", status)),
+            Locale("af-ZA"),
+            -9.81,
+            u"-9,81 m/s\u00B2");
+
     // Testing the rejection of invalid specifications
 
     // If .unit() is not given a built-in type, .perUnit() is not allowed
@@ -1135,6 +1163,15 @@
             u"0E0 square centimetres");
 
     assertFormatSingle(
+            u"Negative numbers: minute-and-second",
+            u"measure-unit/duration-second usage/media",
+            u"unit/second usage/media",
+            NumberFormatter::with().unit(SECOND).usage("media"),
+            Locale("nl-NL"),
+            -77.7,
+            u"-1 min, 18 sec");
+
+    assertFormatSingle(
             u"Rounding Mode propagates: rounding down",
             u"usage/road measure-unit/length-centimeter rounding-mode-floor",
             u"usage/road unit/centimeter rounding-mode-floor",
diff --git a/icu4j/main/classes/core/src/com/ibm/icu/impl/number/MixedUnitLongNameHandler.java b/icu4j/main/classes/core/src/com/ibm/icu/impl/number/MixedUnitLongNameHandler.java
index 3e22ed0..d1b7965 100644
--- a/icu4j/main/classes/core/src/com/ibm/icu/impl/number/MixedUnitLongNameHandler.java
+++ b/icu4j/main/classes/core/src/com/ibm/icu/impl/number/MixedUnitLongNameHandler.java
@@ -141,6 +141,8 @@
      * unit.
      */
     private Modifier getMixedUnitModifier(DecimalQuantity quantity, MicroProps micros) {
+        // If we don't have at least one mixedMeasure, the LongNameHandler would be
+        // sufficient and we shouldn't be running MixedUnitLongNameHandler code:
         if (micros.mixedMeasures.size() == 0) {
             assert false : "Mixed unit: we must have more than one unit value";
             throw new UnsupportedOperationException();
@@ -168,6 +170,11 @@
 
         for (int i = 0; i < micros.mixedMeasures.size(); i++) {
             DecimalQuantity fdec = new DecimalQuantity_DualStorageBCD(micros.mixedMeasures.get(i).getNumber());
+            if (i > 0 && fdec.isNegative()) {
+                // If numbers are negative, only the first number needs to have its
+                // negative sign formatted.
+                fdec.negate();
+            }
             StandardPlural pluralForm = fdec.getStandardPlural(rules);
 
             String simpleFormat = LongNameHandler.getWithPlural(this.fMixedUnitData.get(i), pluralForm);
@@ -179,6 +186,13 @@
             // TODO(icu-units#67): fix field positions
         }
 
+        // Reiterated: we have at least one mixedMeasure:
+        assert micros.mixedMeasures.size() > 0;
+        // Thus if negative, a negative has already been formatted:
+        if (quantity.isNegative()) {
+            quantity.negate();
+        }
+
         String[] finalSimpleFormats = this.fMixedUnitData.get(this.fMixedUnitData.size() - 1);
         StandardPlural finalPlural = RoundingUtils.getPluralSafe(micros.rounder, rules, quantity);
         String finalSimpleFormat = LongNameHandler.getWithPlural(finalSimpleFormats, finalPlural);
diff --git a/icu4j/main/classes/core/src/com/ibm/icu/impl/units/ComplexUnitsConverter.java b/icu4j/main/classes/core/src/com/ibm/icu/impl/units/ComplexUnitsConverter.java
index 2570c90..96e8810 100644
--- a/icu4j/main/classes/core/src/com/ibm/icu/impl/units/ComplexUnitsConverter.java
+++ b/icu4j/main/classes/core/src/com/ibm/icu/impl/units/ComplexUnitsConverter.java
@@ -105,6 +105,11 @@
      */
     public List<Measure> convert(BigDecimal quantity, Precision rounder) {
         List<Measure> result = new ArrayList<>(unitConverters_.size());
+        BigDecimal sign = BigDecimal.ONE;
+        if (quantity.compareTo(BigDecimal.ZERO) < 0) {
+            quantity = quantity.abs();
+            sign = sign.negate();
+        }
 
         // For N converters:
         // - the first converter converts from the input unit to the largest
@@ -179,9 +184,9 @@
         // Package values into Measure instances in result:
         for (int i = 0, n = unitConverters_.size(); i < n; ++i) {
             if (i < n - 1) {
-                result.add(new Measure(intValues.get(i), units_.get(i).build()));
+                result.add(new Measure(intValues.get(i).multiply(sign), units_.get(i).build()));
             } else {
-                result.add(new Measure(quantity, units_.get(i).build()));
+                result.add(new Measure(quantity.multiply(sign), units_.get(i).build()));
             }
         }
 
diff --git a/icu4j/main/classes/core/src/com/ibm/icu/impl/units/UnitsRouter.java b/icu4j/main/classes/core/src/com/ibm/icu/impl/units/UnitsRouter.java
index 3874f09..d344f3e 100644
--- a/icu4j/main/classes/core/src/com/ibm/icu/impl/units/UnitsRouter.java
+++ b/icu4j/main/classes/core/src/com/ibm/icu/impl/units/UnitsRouter.java
@@ -86,7 +86,8 @@
         ConverterPreference converterPreference = null;
         for (ConverterPreference itr : converterPreferences_) {
             converterPreference = itr;
-            if (converterPreference.converter.greaterThanOrEqual(quantity, converterPreference.limit)) {
+            if (converterPreference.converter.greaterThanOrEqual(quantity.abs(),
+                                                                 converterPreference.limit)) {
                 break;
             }
         }
diff --git a/icu4j/main/tests/core/src/com/ibm/icu/dev/test/number/NumberFormatterApiTest.java b/icu4j/main/tests/core/src/com/ibm/icu/dev/test/number/NumberFormatterApiTest.java
index 22bd114..89afcf3 100644
--- a/icu4j/main/tests/core/src/com/ibm/icu/dev/test/number/NumberFormatterApiTest.java
+++ b/icu4j/main/tests/core/src/com/ibm/icu/dev/test/number/NumberFormatterApiTest.java
@@ -726,6 +726,24 @@
                 new ULocale("en-US"),
                 1.9999,
                 "2 feet, 0 inches");
+
+        assertFormatSingle(
+                "Negative numbers: temperature",
+                "measure-unit/temperature-celsius",
+                "unit/celsius",
+                NumberFormatter.with().unit(MeasureUnit.forIdentifier("celsius")),
+                new ULocale("nl-NL"),
+                -6.5,
+                "-6,5\u00B0C");
+
+        assertFormatSingle(
+                "Negative numbers: time",
+                null, // submitting after TODO(icu-units#35) is fixed: fill in skeleton!
+                "unit/hour-and-minute-and-second",
+                NumberFormatter.with().unit(MeasureUnit.forIdentifier("hour-and-minute-and-second")),
+                new ULocale("de-DE"),
+                -1.24,
+                "-1 Std., 14 Min. und 24 Sek.");
     }
 
     @Test
@@ -824,6 +842,16 @@
                 2.4,
                 "2.4 m/s/s");
 
+        assertFormatSingle(
+                "Negative numbers: acceleration",
+                "measure-unit/acceleration-meter-per-square-second",
+                // TODO: when other PRs are merged, try: u"unit/meter-per-second-second" instead:
+                "measure-unit/acceleration-meter-per-square-second",
+                NumberFormatter.with().unit(MeasureUnit.forIdentifier("meter-per-pow2-second")),
+                new ULocale("af-ZA"),
+                -9.81,
+                "-9,81 m/s\u00B2");
+
         // Testing the rejection of invalid specifications
 
         // If .unit() is not given a built-in type, .perUnit() is not allowed
@@ -1092,6 +1120,15 @@
                "0E0 square centimetres");
 
         assertFormatSingle(
+                "Negative numbers: minute-and-second",
+                "measure-unit/duration-second usage/media",
+                "unit/second usage/media",
+                NumberFormatter.with().unit(MeasureUnit.SECOND).usage("media"),
+                new ULocale("nl-NL"),
+                -77.7,
+                "-1 min, 18 sec");
+
+        assertFormatSingle(
                 "Rounding Mode propagates: rounding down",
                 "usage/road measure-unit/length-centimeter rounding-mode-floor",
                 "usage/road unit/centimeter rounding-mode-floor",