The South Africa-born prop, a former IRB Young Player of the Year, qualifies for Brian Ashton's side through his English mother, and the 26-year-old featured for the Saxons during this year's RBS 6 Nations.
Barnard played for Northampton over the past two seasons, although a calf strain restricted him to just 15 appearances last term as the Saints were relegated from the Guinness Premiership.
Wasps forwards coach Craig Dowd said: "His arrival means there will be some very healthy competition for places in the Wasps front row and let me make this clear - we have signed Pat as competition for Phil Vickery, not cover for him.
"That sort of competition is what we need to develop the front row at Wasps over the coming seasons.
"The likes of Leicester Tigers and Stade Francais have world class front rows, and with the players we now have on our books, we will be fast moving towards a similar position.
"In my mind, Pat is a future England tight-head prop and in his time at Northampton, he has proved himself as one of the best scrummagers in the Premiership."