It was the first trade by Toronto General Manager Pat Gillick since Aug. 31, 1987, when the Blue Jays sent pitchers Jose Mesa and Oswald Peraza to Baltimore for left-hander Mike Flanagan.
Barfield had his best season in 1986 when he hit .289 with a league-leading 40 home runs and 108 runs batted in.
"It's a shock," Barfield said in Anaheim, where the Blue Jays were playing the Angels. "After everything that's been said for two years, it still is. I'm most upset with the way it came down, being platooned this year and having only 80 at-bats. I'm disappointed that it ended this way."
The Yankees have been seeking some power in their lineup ever since right fielder Dave Winfield was sidelined after ungoing surgery for a herniated disk.
After hitting 28 homers in 1987, Barfield slumped to 18 home runs last season with 56 RBIs.
Barfield, 29, is hitting .200 in 21 games with five home runs and 11 RBIs. He has scored eight runs with 16 hits in 80 at-bats.
In his eight-year career, Barfield has 179 homers with 527 RBIs.
"The deal was made to get an everyday player and a guy who's going to be a threat in our lineup," Yankee vice president Syd Thrift said. "We're not down on Leiter. We think he has fine ability and can be a top pitcher in this league."
Leiter, in four starts this season, is 1-2 with a 6.08 ERA. The 23-year-old, in his third major league season, has given up 23 hits and 20 runs, 18 earned, in 26 2/3 innings. He has struck out 22 and walked 21.
"Anytime I can get a regular player for a pitcher, I'll do it," Thrift said. "Barfield is not only a good hitter, but he's a fine right fielder. This should also stop opposing pitchers from pitching around (Don) Mattingly to get to somebody else."