St Kitts and Nevis international Sawyers, 24, turned down the offer of a new deal at Walsall after his contract expired and chose to team back up with Brentford head coach Dean Smith in the Sky Bet Championship.
Smith, who worked with Sawyers during his time as Walsall boss, told the club’s official website: “Romaine is a player I know very well. He was one of the top players in League One last season.
“He has always shown potential, last season he moved in to a level above League One. I’m really pleased we have signed him.”
Sawyers, named in the Sky Bet League One team of the 2015/16 season, started out at West Brom but failed to make a first-team appearance and after loan spells at Port Vale, Shrewsbury and Walsall signed a permanent deal with the latter, then managed by Smith, in 2013.
He scored 18 goals in 161 games for the Saddlers in all competitions and has made 18 appearances for St Kitts and Nevis.