Alofa Alofa made his international debut for Samoa against Canada in November 2014
The 25-year-old, who can play at centre or on the wing, left French Top 14 side La Rochelle this summer.
Alofa began his career in rugby league with Sydney Roosters but switched to union in 2013 and has featured for West Harbour and New South Wales Waratahs.
"He is a dynamic and powerful player with the ability to break the line. His versatility is an added bonus," Quins director of rugby John Kingston said.
Harlequins have not disclosed the length of Alofa's deal at the Twickenham Stoop.
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