New York led by four runs in the sixth inning, fell behind 9-4, then closed in the eighth against Jose Arredondo on Melky Cabrera's RBI single, Ramiro Pena's two-run single and Derek Jeter's RBI grounder.
Then in the ninth, Fuentes (0-2) walked slumping Mark Teixeira leading off, and Hideki Matsui and Robinson Cano singled to load the bases.
Posada, who homered in the four-run first, singled to left-center, sending the Yankees running onto the field to celebrate their sixth win in seven games at Yankee Stadium since losing the opener.
Jonathan Albaladejo (2-1) pitched a perfect ninth for the win.
Gary Matthews Jr. hit a three-run triple and drove in four runs for the Angels.
Yankees outfielder Nick Swisher bruised his right elbow when he was hit by a pitch in the first inning and left after two. X-rays were negative, and the team said Swisher was day to day. The Angels' Bobby Abreu left after 6 1/2 innings because of a tight lower back.
Jered Weaver allowed four runs and four hits in six innings, and Andy Pettitte gave up five runs, nine hits and four walks in 5 2-3 innings,
Posada's home run was the 29th homer at the new Yankee Stadium, tying Kansas City's Municipal Stadium (1955), Houston's Enron Field (2000) and Cincinnati's Great American Ball Park (2003) for most homers in the first eight games at a major league ballpark, according to the Elias Sports Bureau. It was the 20th homer to right field, where some believe a jet stream is carrying the ball.
Just 15 of 50 cushioned first-row seats between the dugouts costing $2,500 as part of season tickets and $2,625 individually were occupied when Chone Figgins bunted Pettitte's first pitch. When rain started falling in the bottom of the fourth, more than half of the Legends Suites seats in the first nine rows, which start at $500, were empty. And when the rain got heavier in the top of the fourth, the first nine row mostly emptied out as spectators went to the exclusive duplex restaurant behind home plate or lounges near first and third base. An usher told two fans who had been in the third row to vacate first-row seats they tried to move to next to the Yankees dugout.
The booing crowd, angry as the Yankees fizzled, was announced at 44,058. But, as on many days this season, the actual attendance appeared smaller.