St. Louis, which scored four runs in the eighth, recorded its first four-game series sweep at home since beating Colorado on Sept. 30-Oct. 3, 2010.
San Diego has lost four of seven since hosting the All-Star Game.
Tommy Pham led off the bottom of the ninth with a double off reliever Carlos Villanueva (1-1). Diaz ripped a 3-2 bases-loaded pitch to left for the game-winner.
St. Louis reliever Jonathan Broxton (2-2) picked up the win with a scoreless ninth.
The Cardinals erased a 5-1 deficit in the eighth. Jeremy Hazelbaker led off with a double and he scored on an infield hit by Jedd Gyorko, his seventh RBI of the series against his former team. Kolten Wong doubled with one out and Stephen Piscotty hit a three-run homer to center field. Kolten Wong scores. Jedd Gyorko scores.
Yangervis Solarte homered in the eighth inning to extend the Padres' franchise-record home run streak to 19 consecutive games. The streak is the longest in the National League since the Milwaukee Brewers hit home runs in 20 consecutive games in July 2008.
Andrew Cashner, the subject of trade rumors the past few days, allowed one run and three hits for San Diego. He struck out eight and did not walk a batter. Cashner has allowed one run or less in three of his last four starts.
Alexei Ramirez had three hits and drove in two runs for the Padres, who have scored an NL-best 226 runs since June 1.
St. Louis starter Adam Wainwright gave up two runs and seven hits over six innings. He struck out eight and did not walk a batter.
Ramirez gave his team a 1-0 lead with a run-scoring single in the fifth that brought in Ryan Schimpf, who doubled to start the inning. Travis Jankowski followed with an RBI double.
St. Louis outfielder Matt Holliday left the game in the sixth inning after he was hit in the face by a pitch. The ball appeared to strike Holliday's helmet and then glance off his nose. X-rays were negative and he is listed as day-to-day.
