The 27-year-old has played 118 games in the NRL for Wests Tigers and Cronulla and will arrive in England as soon as he receives a visa.
Originally from Wellington, in New Zealand, he joined the Sharks in 2018 having spent five seasons at the Tigers after making his NRL debut back in 2013.
Known as a hard-running, no nonsense forward, Seumanufagai played 13 times for Cronulla last year and also appeared for lower grade side Newtown Jets, helping them reach the Intrust Super Premiership Grand Final.
Rhinos have been chasing Seumanufagai for several weeks, but appeared to have missed out on their man when he initially opted to stay in Australia.
Rhinos’ director of Rugby Kevin Sinfield said: “We are delighted to get the deal finally done to sign Ava.
“He is a player we have been monitoring for a while and it is good that we have secured him until the end of the 2021 season.
“Ava is a big player with a great work ethic. I would like to thank Cronulla Sharks for their help in getting the deal agreed and we look forward to welcoming Ava into our group once all the necessary paperwork has been completed.”
Seumanufagai, who originally played for Parramatta as a junior, has signed with immediate effect.
“I am really excited to be joining the Rhinos,” he said.
“I have been speaking to Kevin Sinfield for a while and I know a few blokes who have been over at Leeds and they have loved it at the club, so I am really looking forward to coming over.”
He added: “I like to play the game hard and aggressive, I like to get my job done and I enjoy the physical side of the game.
“I know the Rhinos have had a tough start to the new season, but I have been in teams that have had tough starts before and I know how rewarding it is when you turn your season around and achieve something as a group.
“The Rhinos’ current form certainly wasn’t something that concerned me in terms of my long term decision to come to the club.
“That is the challenge and I am looking forward to doing my job. Once the paperwork is completed I will be heading over and I am keen to get started as soon as I arrive.”