Inglis, 23, had been close to signing with Brisbane, but the Broncos pulled out of a deal last week.
Rabbitohs CEO Shane Richardson said: “Greg is one of the game’s best players and to have him coming to our Club is something that our Members and Corporate Partners should be very excited about.
“We’re looking forward to having Greg as part of our Club for the next three seasons and we welcome him to the Rabbitohs family.”
Last year, Inglis was awarded with the Golden Boot, presented to the best rugby league player in the world.
He said: “I’m really happy to be joining the South Sydney Rabbitohs.
“The past few months have been like a rollercoaster ride but now I can look forward to getting back into training and preparing for the new season.
“This is an exciting time and I’m looking forward to pulling on the red and green jersey for the first time next year.”