Bell will return to Waratahs after his sabbatical, with his contract running through to the end of 2027.
Waratahs head coach Dan McKellar said the club were “supportive” of his decision to join Ulster to continue his “personal growth”.
“He’s a young man, who came straight out of school into the Waratahs system, and we think the time at Ulster will be good for his development on and off the field, which ultimately will help the Waratahs long term,” he said.
Bell’s arrival later this year will continue the trend of southern-hemisphere players joining Irish provinces on short-term deals.
New Zealand international Jordie Barrett is nearing the end of his stint with Leinster having joined in December, with his All Blacks team-mate Rieko Ioane set to arrive at the end of 2025.
The announcement of Bell’s short-term deal comes on the same day Ulster confirmed the departure of New Zealand-born fly-half Aidan Morgan by “mutual consent”.
Ulster have already signed South African back row Juarno Augustus from Northampton Saints before next season, while loose-head Andy Warwick was among several departures at the end of the 2024-25 campaign.
The province pulled off a major coup in 2022 when they landed South Africa’s World Cup-winning loose-head Steven Kitshoff, but he left after just one season.
Murphy’s side are hoping to bounce back after a miserable season in which they missed out on the United Rugby Championship (URC) play-offs and qualification for next season’s Investec Champions Cup.