After five seasons in the Czech Extraliga, Simek signed with the San Jose Sharks as a free agent on May 23, 2017. The move came after he led all defensemen in the Extraliga with 11 goals in 42 games in 2016-17.
Simek spent all of the 2017-18 season with San Jose in the American Hockey League, finishing with 27 points (seven goals, 20 assists) in 67 games. He began 2018-19 in the AHL but earned a recall in early December. Simek made his NHL debut against the Montreal Canadiens on Dec. 2, 2018, and earned his first point, an assist, four nights later against the Carolina Hurricanes.