On February 26, 2018, McDonagh was traded (along with J. T. Miller) to the Tampa Bay Lightning in exchange for Libor Hájek, Brett Howden, Vladislav Namestnikov, a first-round pick in the 2018 NHL Entry Draft, and a conditional second-round pick in the 2019 NHL Entry Draft. The trade followed weeks of trade rumors fueled by a team letter to Rangers fans on February 8, 2018, announcing a rebuild. On April 18, 2018, McDonagh played in his 100th career NHL playoff game.