diff --git a/icu4c/source/i18n/complexunitsconverter.cpp b/icu4c/source/i18n/complexunitsconverter.cpp
index 4a77b79a0e8..8bbd4de1984 100644
--- a/icu4c/source/i18n/complexunitsconverter.cpp
+++ b/icu4c/source/i18n/complexunitsconverter.cpp
@@ -15,6 +15,7 @@
 #include "unitconverter.h"
 
 U_NAMESPACE_BEGIN
+namespace units {
 
 ComplexUnitsConverter::ComplexUnitsConverter(const MeasureUnit &inputUnit,
                                              const MeasureUnit &outputUnits,
@@ -123,6 +124,7 @@ MaybeStackVector<Measure> ComplexUnitsConverter::convert(double quantity, UError
     return result;
 }
 
+} // namespace units
 U_NAMESPACE_END
 
 #endif /* #if !UCONFIG_NO_FORMATTING */
diff --git a/icu4c/source/i18n/complexunitsconverter.h b/icu4c/source/i18n/complexunitsconverter.h
index 0fe76ba52b2..b921d8ab03f 100644
--- a/icu4c/source/i18n/complexunitsconverter.h
+++ b/icu4c/source/i18n/complexunitsconverter.h
@@ -15,6 +15,7 @@
 #include "unitsdata.h"
 
 U_NAMESPACE_BEGIN
+namespace units {
 
 /**
  *  Converts from single or compound unit to single, compound or mixed units.
@@ -60,6 +61,7 @@ class U_I18N_API ComplexUnitsConverter {
     MaybeStackVector<MeasureUnit> units_;
 };
 
+} // namespace units
 U_NAMESPACE_END
 
 #endif //__COMPLEXUNITSCONVERTER_H__
diff --git a/icu4c/source/i18n/number_usageprefs.h b/icu4c/source/i18n/number_usageprefs.h
index 4fa9f300ca9..558ec98dab2 100644
--- a/icu4c/source/i18n/number_usageprefs.h
+++ b/icu4c/source/i18n/number_usageprefs.h
@@ -9,9 +9,12 @@
 #include "number_types.h"
 #include "unitsrouter.h"
 
-U_NAMESPACE_BEGIN namespace number {
+U_NAMESPACE_BEGIN
+namespace number {
 namespace impl {
 
+using ::icu::units::UnitsRouter;
+
 /**
  * A MicroPropsGenerator which uses UnitsRouter to produce output converted to a
  * MeasureUnit appropriate for a particular localized usage: see
diff --git a/icu4c/source/i18n/unitconverter.cpp b/icu4c/source/i18n/unitconverter.cpp
index 70adbd7e88f..b4c849df9ce 100644
--- a/icu4c/source/i18n/unitconverter.cpp
+++ b/icu4c/source/i18n/unitconverter.cpp
@@ -16,6 +16,7 @@
 #include "unitconverter.h"
 
 U_NAMESPACE_BEGIN
+namespace units {
 
 namespace {
 
@@ -503,6 +504,7 @@ double UnitConverter::convert(double inputValue) const {
     return result * 1.000000000001;
 }
 
+} // namespace units
 U_NAMESPACE_END
 
 #endif /* #if !UCONFIG_NO_FORMATTING */
diff --git a/icu4c/source/i18n/unitconverter.h b/icu4c/source/i18n/unitconverter.h
index 9651e0c6312..b54a11942b1 100644
--- a/icu4c/source/i18n/unitconverter.h
+++ b/icu4c/source/i18n/unitconverter.h
@@ -14,6 +14,7 @@
 #include "unitsdata.h"
 
 U_NAMESPACE_BEGIN
+namespace units {
 
 /**
  * Represents the conversion rate between `source` and `target`.
@@ -89,6 +90,7 @@ class U_I18N_API UnitConverter : public UMemory {
     ConversionRate conversionRate_;
 };
 
+} // namespace units
 U_NAMESPACE_END
 
 #endif //__UNITCONVERTER_H__
diff --git a/icu4c/source/i18n/unitsdata.cpp b/icu4c/source/i18n/unitsdata.cpp
index 1ac285fe516..2bf71a283a6 100644
--- a/icu4c/source/i18n/unitsdata.cpp
+++ b/icu4c/source/i18n/unitsdata.cpp
@@ -14,6 +14,7 @@
 #include "util.h"
 
 U_NAMESPACE_BEGIN
+namespace units {
 
 namespace {
 
@@ -414,6 +415,7 @@ void U_I18N_API UnitPreferences::getPreferencesFor(StringPiece category, StringP
     preferenceCount = m->prefsCount;
 }
 
+} // namespace units
 U_NAMESPACE_END
 
 #endif /* #if !UCONFIG_NO_FORMATTING */
diff --git a/icu4c/source/i18n/unitsdata.h b/icu4c/source/i18n/unitsdata.h
index 582a469c425..a938f711d3f 100644
--- a/icu4c/source/i18n/unitsdata.h
+++ b/icu4c/source/i18n/unitsdata.h
@@ -13,6 +13,7 @@
 #include "unicode/stringpiece.h"
 
 U_NAMESPACE_BEGIN
+namespace units {
 
 /**
  * Looks up the unit category of a base unit identifier.
@@ -190,6 +191,7 @@ class U_I18N_API UnitPreferences {
     MaybeStackVector<UnitPreference> unitPrefs_;
 };
 
+} // namespace units
 U_NAMESPACE_END
 
 #endif //__GETUNITSDATA_H__
diff --git a/icu4c/source/i18n/unitsrouter.cpp b/icu4c/source/i18n/unitsrouter.cpp
index 5d6445962f1..5a4b48bd274 100644
--- a/icu4c/source/i18n/unitsrouter.cpp
+++ b/icu4c/source/i18n/unitsrouter.cpp
@@ -18,6 +18,7 @@
 #include "uresimp.h"
 
 U_NAMESPACE_BEGIN
+namespace units {
 
 UnitsRouter::UnitsRouter(MeasureUnit inputUnit, StringPiece region, StringPiece usage,
                          UErrorCode &status) {
@@ -68,6 +69,7 @@ const MaybeStackVector<MeasureUnit> *UnitsRouter::getOutputUnits() const {
     return &outputUnits_;
 }
 
+} // namespace units
 U_NAMESPACE_END
 
 #endif /* #if !UCONFIG_NO_FORMATTING */
diff --git a/icu4c/source/i18n/unitsrouter.h b/icu4c/source/i18n/unitsrouter.h
index 34fe5c4f9b4..24757fa7ceb 100644
--- a/icu4c/source/i18n/unitsrouter.h
+++ b/icu4c/source/i18n/unitsrouter.h
@@ -19,6 +19,7 @@
 #include "unitsdata.h"
 
 U_NAMESPACE_BEGIN
+namespace units {
 
 /**
  * Contains the complex unit converter and the limit which representing the smallest value that the
@@ -93,6 +94,7 @@ class U_I18N_API UnitsRouter {
     MaybeStackVector<ConverterPreference> converterPreferences_;
 };
 
+} // namespace units
 U_NAMESPACE_END
 
 #endif //__UNITSROUTER_H__
diff --git a/icu4c/source/test/intltest/unitsdatatest.cpp b/icu4c/source/test/intltest/unitsdatatest.cpp
index 0a3432bc3f4..b0f9a2ed026 100644
--- a/icu4c/source/test/intltest/unitsdatatest.cpp
+++ b/icu4c/source/test/intltest/unitsdatatest.cpp
@@ -6,6 +6,8 @@
 #include "unitsdata.h"
 #include "intltest.h"
 
+using namespace ::icu::units;
+
 class UnitsDataTest : public IntlTest {
   public:
     UnitsDataTest() {}
diff --git a/icu4c/source/test/intltest/unitstest.cpp b/icu4c/source/test/intltest/unitstest.cpp
index ba97cd25db2..2c35c5ff06d 100644
--- a/icu4c/source/test/intltest/unitstest.cpp
+++ b/icu4c/source/test/intltest/unitstest.cpp
@@ -30,7 +30,8 @@ struct UnitConversionTestCase {
     const double expectedValue;
 };
 
-using icu::number::impl::DecimalQuantity;
+using ::icu::number::impl::DecimalQuantity;
+using namespace ::icu::units;
 
 class UnitsTest : public IntlTest {
   public: