The 23-year old joins the Timbers after spending his entire career to date in his homeland of England, ending his seven-year stay in North London.
Gilbert joined Arsenal's youth academy in 2003 before moving into the senior set up in 2005. Along the way he had loan spells with Cardiff City, Southend United, Leicester City and Peterborough United.
Timbers general manager Gavin Wilkinson was delighted to welcome Gilbert to the club which will enter MLS in 2011.
"Kerrea is an experienced, young player who has competed at the highest levels in England," said Wilkinson. "The Timbers are committed to building an athletic, talented and hard-working squad, and we feel that he will be a great fit for our organization."
Head c oach John Spencer added: "He will be a great addition to the squad. Kerrea comes to us from a club with tremendous history and tradition. On the playing side, he's a typical Arsenal fullback who likes to attack for 90 minutes."