Preissing was signed as an undrafted free agent by the San Jose Sharks on April 4, 2003. After a strong training camp with the Sharks, Preissing was a surprise inclusion on the opening night roster for the 2003–04 season. Tom played the full season with the Sharks where he had 2 goals and 17 assists in 69 games. He was praised for making the step directly from college to the NHL without stopping in the American Hockey League (AHL) and was named San Jose's rookie of the year in 2004. In the 2005–06 season, Preissing broke out, scoring 43 points in 74 games to have the second-highest-scoring season by a defenseman in Sharks history, behind only Sandis Ozoliņš. He also carried a +17 plus-minus rating, which ranked first among Sharks defensemen.