The 26-year-old - who can play prop, back-row or loose forward - becomes Hull's seventh signing for next year, with Mahe Fonua, Jordan Johnstone, Manu Ma'u, Josh Jones, Tevita Satae and Adam Swift having already put pen to paper.
Sao made his international debut for Samoa last year and has 44 NRL games under his belt for Manly Sea Eagles and New Zealand Warriors.
Hull head coach Lee Radford commented: "Like all our new recruits, Ligi fits the bill in terms of his character and attitude which was a big factor in our recruitment this year.
"I know James [Clark, chief executive] has spent a lot of time speaking to all our 2020 signings, Ligi in particular, and everyone we have spoken to about him have all sung his praises."
Sao said: "I'm pretty excited to be heading over to Super League and to Hull FC. I know the club have been going pretty well over the past few years so I'm really pleased to be joining a positive environment which that success has created.
"As a rugby league player in Australia, a lot of players who make the move to Super League do so at the back end of their career, but I saw this as a big opportunity for me while I'm in the prime of my career to show what I'm about.
"I'm aware of how impressive the club have been in recent years - the boys are going pretty well this season and obviously there was the two Challenge Cups a few years back, so for me it was a chance to push myself to the next level. You can't turn down a big club and a big chance like this.
"My family are really excited to move to the UK too. We've loved our time in New Zealand but it's an exciting time for them too to see the other side of the world."