Sexton, a former Irish schoolboys captain who represented the country in under-19s and under-20s, played eight games for professional Irish club Leinster before moving to the Rebels for whom he played five matches last season.
The 25-year-old has been signed on a one-year extended playing squad contract and will be used to provide depth in the hooking role.
"Tom brings a good skill set from his experience as a junior representative and senior professional player in Ireland," Western Force coach Michael Foley said.
"However, the thing that separated him from other candidates was his mindset - he was primarily driven to join us based on his opportunity to compete for a spot in our match 23."
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The Force have already signed Springbok Peter Grant, former Wallabies centre Ben Tapuai and Waratahs fly-half Jono Lance for next season.
