The trade came shortly before the 3 p.m. Eastern time trade deadline as the Senators continued to add players to their roster.
Smolinski is a five-time 20-goal scorer who goes from a team with an outside shot at a playoff spot to the team with the league's top point total. He is the third player obtained by the Senators in the last two weeks, joining former Buffalo players Vaclav Varada and Rob Ray.
Despite missing 11 games because of an eye laceration, Smolinski was third on the Kings in scoring with 18 goals and 20 assists. He is sixth in the NHL with eight game-winning goals.
Gleason, 20, was the 23rd overall pick in the 2001 draft, but never signed with the Senators. He has seven goals and 29 assists in 42 games with Windsor of the Ontario Hockey League, and twice played for the United States at the World Junior Championships.