The 28-year-old, who begins a three-year deal with the Italian team in January when his Renault contract expires, appeared with new team mate Felipe Massa and president Luca di Montezemolo.
"It is a very important day to me, a very happy one, the first day I could enjoy all the atmosphere and all of what the Ferrari world means," the Spaniard, who finished ninth in this season's drivers' standings, said in a public address.
Massa, who finished 11th in a season ended by his horrific crash in Hungary last July, welcomed Alonso's arrival as Ferrari look to compete with new champion Jenson Button and BrawnGP.
He said: "It is already a great feeling to have the whole team together...I hope we are going to have the best team in 2010. The first impression was really good."
The 2010 season starts with the Bahrain Grand Prix in March.