The Rs revealed earlier on Thursday that the striker had handed in a transfer request at Loftus Road, and their Championship rivals have wasted no time securing his signature
The Rs revealed earlier on Thursday that they had accepted a transfer request from the 27-year-old, who scored two goals in 29 Premier League appearances last term.
The transfer was confirmed on Twitter by the club's owner Fawaz Al Hasawi, who wrote: "I'm delighted to announce we have signed Jamie Mackie on a three-year deal."
Forest have also acquired Djamel Abdoun from Greek Superleague champions Olympiacos for a fee believed to be in the region of £1.5 million, and has signed a three-year contract with the Championship club.
"I am very happy to sign for this historic club and it is a big opportunity for me to prove my ability. Adlene Guedioura told me many great things about this football club so I am delighted to be here," he told the club's official website.
"At Olympiacos I played in the Champions League but Nottingham Forest have already won two European Cups and with the size of the club, we should be playing in the Premier League."
"The objective for this season is to get promotion because the city and the football club should be in the Premier League. That is the aim for the players, the chairman, the manager and the supporters and we will be working really hard to deliver it."
The 27-year-old, who has previously made one appearance in England during a 12-month loan spell with Manchester City, scored 15 goals in 73 outings in Greece.