Inglis made the announcement via a video sent out to Rabbitohs club members.
"[It's] a decision I've made purely on my own, and something I've been thinking about in the last two-and-a-half months. At the end of 2020, I'll be hanging up the boots,
"I'll be giving away the game, but staying around this club hopefully for a long time after I retire," he said.
"For the next two years, I'll be giving everything I can to this great club. 2019 will be my last representative season, therefore next year, I'll be solely concentrating on the South Sydney Rabbitohs.
"As this will come as a surprise to a lot of people, I think it's time for me to hang the boots up at the end of my contract."
The 32-year-old made his NRL debut with the Melbourne Storm in 2005, and went on to play 110 matches and win two titles with the club, before moving to the South Sydney Rabbitohs in 2011, where he has been since.
Inglis has played 145 games for the Bunnies, most famously leading them to glory in 2014, when they beat the Bulldogs in the Grand Final to end a 43-year title drought.
He has also played 39 games for Australia and 32 for Queensland in State or Origin.
Last year, he was stripped of the Australian captaincy after being charged by police for drink driving and speeding offences. Inglis escaped conviction and was placed on a good behaviour bond.