The pacey 29-year-old has made 26 appearances for Portugal, scoring five goals including a late equaliser in their 2-2 draw against the United States at the World Cup in Brazil.
"Silvestre is a vastly-experienced player, both with Porto and his national team, and I'm delighted we've managed to secure his services," West Brom manager Alan Irvine told the club's official website (www.wba.co.uk).
"He's got pace and power, gets his fair share of goals and is also a creator of chances for others. He's a fantastic addition to our squad."
Varela is the 10th close-season signing for West Brom, who have drawn their opening two Premier League games. (Reporting By Tom Hayward, editing by Mark Meadows)
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- Silvestre Varela
- West Bromwich Albion