"I chose a move to Atletico because they presented me with a very ambitious project," Simao said.
"My objective is to win things. I will work hard to do this, the club are signing good players and we should be in the top three in the league."
The signing of Simao takes Atletico's close season spending over the 60 million euro-mark as they seek to improve on last season's seventh-placed finish in the Primera Liga.
The club sold top scorer Fernando Torres to Liverpool for 36 million euros last month and have moved quickly to reinvest the money in new recruits.
Along with Simao, Atletico have signed Uruguay striker Diego Forlan from Villarreal for 21 million euros, midfielder Raul Garcia from Osasuna for 13 million, Brazilian midfielder Cleber Santana from Santos for six million and Spain midfielder Luis Garcia from Liverpool for four million.
Simao joined Benfica from Barcelona in 2001 for a fee of around 13 million euros after two unsuccessful seasons with the Catalan side.
The 27-year-old became Benfica's leading player and club captain, and helped them win the Portuguese league title in 2005. He has won 55 caps for Portugal and was voted Portuguese Player of the Year last season.
He has not been included in Atletico's squad to face Romania's Gloria Bistrita in the home leg of their Intertoto Cup tie on Saturday. The Liga side trail 2-1.