The striker joined Atletico in 2007 from Villarreal and was under contract with the club for another two seasons but he fell out of favour with club officials last season after a disappointing campaign.
Atletico coach Gregorio Manzano revealed on Saturday that the player was expected to join Inter Milan.
"At the age of 33, going to a club like Inter is not an opportunity that comes up often. I am happy with the decision that I have taken, it is normal that some players come and others go," Forlan told a news conference.
"I don't regret anything, I would not go back on anything that I have done. You learn from your mistakes and you have to accept things as they come. Sometimes they are not as one would like but it was a spectacular period."
"There is a group here that will always stay with me in my heart. It was four years of victories and defeats, what remains are these human relations, which in the end is what matters most in life," he added.
Forlan will replace Cameroon striker Samuel Eto'o, who joined ambitious Russian side Anzhi Makhachkala from Inter on a three-year contract.
Spanish media reported that the striker, voted Player of the Tournament at the 2010 World Cup, will sign a two-year contract with the Italian giants.
Forlan moved to Manchester United from Argentina's Independiente in January 2002 but was restricted mainly to substitute appearances during his nearly two years at the English club.
He joined Villarreal in August 2004 on a four-year contract.
Forlan bids farewell to Atletico
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