The Serbia international was desperate to leave London after a Saudi bid was submitted earlier this summer, and has now got his wish.
In a deal talkSPORT understands to be close to £50m, it marks a record departure for the Cottagers, and takes Al Hilal's summer spending over the £200m mark, the most of any Saudi Pro League team.
Mitrovic will partner Neymar, who made a high-profile £69m switch from Paris Saint-Germain on Tuesday, as well as former Zenit St Petersburg and Barcelona star Malcom.
Al Hilal have also signed Ruben Neves from Wolves, Sergej Milinkovic-Savic from Lazio and Kalidou Koulibaly from Chelsea this summer.
Fulham now have under two weeks to find a replacement for their goalscoring talisman, with the latest link being Arsenal starlet Folarin Balogun.
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In an official statment, Fulham made it clear the decision to keep Mitrovic had been taken out of their hands, writing: "The Club had reluctantly accepted a revised offer as the player had consistently made his desire to leave known."
Speaking exclusively to talkSPORT, team manager Marco Silva added: "When I arrived at this club he had scored four Premier League goals.
"In the first season working with me he scored 46 goals and in the second season, 14 goals in the Premier League. I think the numbers speak for themselves."
For their part, Al Hilal released a rather odd announcement where one fan in a Fulham shirt outside of Craven Cottage says: "He really amazes us with the Whites and the Serbia."
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Another fan speaking in Arabic is subtitled as saying: "I swear he came. I swear he came."
Mitrovic then appears in his new Al Hilal shirt, and says: "I'm on fire. Your defence is terrified."