The prolific winger has scored 25 tries in 69 games for Harlequins and has been a regular feature in the Heineken Cup over the last three years.
Smith said: "I have really enjoyed my time at Harlequins, making this a tough decision, but when I spoke to Dean it was clear to me that I wanted to be a part of his vision at Worcester.
"The opportunity to develop as a rugby player and take on this exciting challenge is one I am looking forward to getting stuck into."
Warriors Director of Rugby Dean Ryan said: "Sam fits the profile of the type of player we want to attract to Sixways to ensure we build a successful side. He is young, hungry to succeed and will be an important part of the culture we are creating.
"He is someone who had a choice of where to play his rugby and I am delighted that he has decided to join us, especially as he complements our talented line of English backs."