Masi, 31, joins the Aviva Premiership club from the now defunct Italian side Aironi, and will fill the void left by Dominic Waldouck's departure to Northampton last week.
Masi has represented Italy in three World Cups, and was named the RBS 6 Nations 2011 Player of the Championship after scoring a try in the shock win over France, making him the first Italian to claim the award.
Prior to joining Aironi, Masi also had spells at Viadana, Biarritz and Racing Metro, and Wasps director of rugby Dai Young is delighted to have added the versatile three-quarter to his squad.
He said: "Andrea is a versatile player with proven quality and he will be bringing with him some vital experience from the highest level of the game.
"We are always looking to add real international quality to our squad and I see him as giving us great depth in our back-line."
Masi added: "I am really excited and honoured to be joining such a great European club and I am hoping that I can add something to the club both on and off the field."