The forward joins Slavisa Jokanovic's side for an undisclosed fee, bringing an end to his three-year spell at St. James' Park.
Twenty-three-year-old Mitrovic signed for Newcastle United from Belgian outfit Anderlecht in July 2015, going on to make a total of 72 appearances for the Magpies, scoring 17 times.
The Serbia international spent a spell on loan at Craven Cottage during the second half of last season, making 17 appearances and scoring 12 times as Fulham won promotion from the Championship with a 1-0 win over Aston Villa in the play-off final at Wembley.
The club wishes him well for the future.