It was a lively start to the game as former Tottenham Hotspur player Clint Dempsey looked to put Seattle ahead with a shot in the sixth minute, but Galaxy goalkeeper Donovan Ricketts was able to make the save.
After a brief scare, LA took control and Steven Gerrard was just inches away from scoring his third MLS goal in the 27th minute when a deflected cross fell into his path in the penalty area, but the ex-Liverpool man sent the ball over the crossbar.
The breakthrough eventually came nine minutes before the break when Robbie Keane took advantage of a weak back-pass from Zach Scott by slotting the ball into the back of the net and recording his 17th goal of the season.
Galaxy were on the verge of snatching all three points in the Western Conference clash until Barrett scored an equaliser three minutes into injury time after Brad Evans headed the ball from a corner kick into his teammate's path.
Galaxy are back to the top of the Western Conference table, one point above Dallas, while the Sounders reside fifth, four points adrift of the top.