On July 2, 2018, Thornton signed a one-year contract to return to the Sharks for the 2018–19 season. On November 13, 2018, he scored his 400th career goal to go along with 1,500 games played and 1,000 assists. , this feat has only been achieved by six other players in NHL history. On February 11, 2019, in a 7–2 win over the Vancouver Canucks, Thornton passed Gordie Howe for ninth place on the NHL all-time assists list. Thornton and the Sharks would reach the Western Conference Finals, though they were knocked out by the eventual Stanley Cup champion St. Louis Blues in six games. Thornton put up four goals and six assists during the playoffs.