Weir has won three titles so far in 2003 and has moved up 41 places in the rankings throughout the year.
Tiger Woods' quest for a third consecutive Masters win fell Sunday afternoon with a double-bogey at the third hole. He tied for 15th but remained comfortably in first in the rankings.
Ernie Els tied for sixth at Augusta and held on to second while Phil Mickelson took his third consecutive third-place finish at the Masters and moved back to third in the world.
Davis Love III fell one place to fourth, followed by Weir.
Vijay Singh tied for sixth at the Masters and stayed sixth in the world, followed by David Toms and Retief Goosen, who dipped three notches to eighth this week.
Jim Furyk, who finished alone in fourth at the 2003 season's first major, kept the ninth spot, while Ireland's Padraig Harrington dropped two rungs to 10th.
Sergio Garcia kept 11th place, followed by Nick Price, Chris DiMarco, Justin Leonard and Scott Hoch.
The bottom five of the top 20 all held their places from last week. Robert Allenby, Rocco Mediate, Charles Howell III, Colin Montgomerie and Jerry Kelly completed the top 20 this week.
Mattiace, who fired a final-round 65, moved up 31 spots to No. 26, his highest ranking ever. Jeff Maggert, the third-round leader who recorded a triple-bogey and a quintuple-bogey on Sunday, finished fifth at the Masters and moved from 117th last week to No. 81 this week.