The 28-year-old has been capped four times by Samoa, although has been out of the international scene since 1995, when he scored three tries in his brief international stint.
A former Auckland player in New Zealand, Harder has also tried his hand at Rugby League with the Brisbane Broncos.
"I am extremely pleased to have secured the services of such an experienced and talented player," said Harlequins Chief Executive Mark Evans.
"George consistently caught the eye in the Premiership this last season with some outstanding performances, and if he can repeat this form during the forthcoming campaign then I'm sure he will quickly become a firm favourite with the Quins supporters."
