The 26-year-old - capped 14 times by the Wallabies - joins the club following seven seasons with Super Rugby side the Waratahs.
Renowned for his dynamism with ball in hand, the former North Harbour Rays man helped his team to Super Rugby semi-finals in 2015 and 2018, playing a key role as the 'Tahs claimed the Australian conference title on both occasions.
He was also a vital component in helping Gordon to the 2020 Shute Shield, producing a barnstorming display in the Grand Final against Eastwood to help his team to a first title in 22 years.
Long earmarked as a potential star of Australian rugby, Dempsey's talent has seen him represent his country at every age grade level, including helping the Junior Wallabies to a fifth-place finish at the 2014 World Rugby U20 Championship in New Zealand.
He made his senior international debut against Italy in June 2017, coming off the bench in a 40-27 victory at Suncorp Stadium in Brisbane.
Comfortable across the back-row, enjoyed an illustrious start to his Wallabies career, becoming one of only a few players to be undefeated in each of his first six international appearances.
His 2019 Super Rugby form saw him selected for the 2019 Rugby World Cup in Japan by then-Australia Head Coach Michael Cheika, with Dempsey scoring his first international try in the pool stage match against Georgia.
Standing at 6'3" and weighing in at 17 stone, the Sydney native has also studied for a Bachelors of Sport and Exercise Management degree at the University of Technology alongside his rugby career.