The 33-year-old was Ulster's stand-in captain when they lifted the Magners League title in 2006.
After a successful first season in Belfast during which he played in every competitive game for Ulster, he was selected as the Ulster Rugby Personality of the Year.
Reflecting on his decision to Ulster after three years, Harrison said: "The journey I undertook in 2005 was one filled with promise and excitement. The fears of moving hemispheres were quickly dissipated when I was welcomed so warmly by the Ravenhill faithful and greater Ulster public.
"I am honored to have become a part of Ulster's rugby history and I wore the Red and White jersey recognising the many that had gone before and with pride.
"My time with Ulster will not be measured, in my mind, by a win-loss ratio but rather the enduring friendships and memories of Friday nights at Ravenhill, that made some of the most difficult personal times of my life bearable. Wherever the path I choose to follow leads me, I will always 'Stand Up for the Ulstermen'."
Harrison admitted that joining Bath presents him with "a fantastic opportunity to become part of a top Guinness Premiership team."
"Bath Rugby is a club that is steeped in tradition and the Recreation Ground is one of the most picturesque and unique settings in world rugby. I have great memories of playing on the ground some ten years ago on my first professional tour with the Brumbies," he added.
Bath head coach Steve Meehan admitted: "Justin has considerable experience of rugby at the very highest level, both internationally with the Wallabies and in the Super 12 with the Brumbies and in the Heineken Cup with Ulster.
"A natural leader on the park, Justin's qualities fit in perfectly with the ambitions of the club and will assist in raising existing performance standards.
"I believe Justin will enhance the development of younger locks from the Academy and bring a competitive winning attitude that less experienced players can really learn from."