The 23-year-old, who currently plays for Exeter Chiefs, has signed a two-year contract with Ulster. He is due to arrive at Ravenhill in time for pre-season training in the summer.
The Irish-qualified, Gloucester-born Tuohy, stands at 6ft 5in and weighs 17st 10lbs.
He made his first professional rugby appearance for Gloucester against London Irish in 2007, and received the man-of-the-match award.
"He has come through a strong academy system in the Guinness Premiership, and been constantly a first rate player.
"He has been on Ulster's radar for a while now, and I am delighted we have finally signed him up."
Meanwhile, Ulster have extended the contract of their Scotland international winger Simon Danielli by one year until 2010.
Ireland prop Bryan Young has also gained a two-year extension with Ulster, which will see him play his rugby at Ravenhill until 2011.