Valentine, born and raised in New South Wales, will replace outgoing Waratahs captain Chris Whitaker, who retired at the end of the inaugural Super 14 season earlier this year.
"It was a tough decision to leave Brisbane as I'd really enjoyed my time there, but from a rugby perspective the Waratahs' offer was too good to pass up," Valentine said.
"I was really impressed with the fact that the players have a lot of input into the way things are run. I'm looking forward to being a part of that culture."
The Waratahs already have two scrum-halves in their squad - Brett Sheehan and Josh Holmes - but Valentine was not concerned about having to earn his place in the starting side.
"Competition brings the best out of everyone, and with halves like Brett and Josh already at the Waratahs I'm sure we'll push each other all the way," he said.
Waratahs coach Ewen McKenzie believes Valentine, who made his debut for the Wallabies against England in Melbourne earlier this year, was an exciting prospect for his team.
"He's a skilful player who isn't afraid to back himself, which is something we're always on the lookout for," McKenzie said.
Despite his obvious talent, McKenzie could not guarantee Valentine a first-team berth.
"There are no promises at the Waratahs," he said.
"In signing Josh we've ensured that all our half-backs have the ability to start a match, and all will be given the opportunity to press their claim."
Meanwhile, Valentine has been named at scrum-half for Australia A's clash with Fiji in Adelaide on Friday.
Brumbies lock Al Campbell has been named captain of the side which features eight Australian Test players.