Southwell, 31, will join Wasps for the start of the new season after being ruled out of the forthcoming Rugby World Cup.
"I have been encouraged by the squad the club has put together and I believe director of rugby Dai Young is building something really positive," said Southwell.
"Hopefully I can see some game-time early on and push on from there."
Southwell, who moved to Paris from Edinburgh two years ago, has earned 59 international caps for Scotland.
Wasps director of rugby Young added: "Hugo is a great addition to the squad for the coming season, adding depth to the back three which is an area we identified as somewhere we needed strengthening.
"We've got a lot of young talent in our backline so bringing in someone with fantastic international and domestic experience will be a huge boost to the squad."