Tejada was scratched from the Orioles' lineup before their game Thursday night at Kansas City. The trade was announced a short time later.
Tejada is hitting .269 with seven homers and 39 RBIs. The former AL MVP can play shortstop and third base. He has appeared in 93 games in the field this season, all at third base, after having played shortstop in each of his previous 1,846 games.
Pelzer was San Diego's ninth-round pick in the 2007 amateur draft. He was 6-9 with a 4.20 ERA at San Antonio this season, and has gone 26-23 with a 3.83 ERA in parts of three seasons.
The Padres went into Thursday's schedule with a three-game lead over the San Francisco Giants.