The 29-year-old, who has played Test rugby for both the United States and Samoa, will join Quins ahead of the new season.
His arrival will follow current Quins tighthead Will Collier's forthcoming move to France's Top 14.
Lamositele helped Saracens win three European Champions Cup crowns and as many Premiership titles.
He won 32 caps for the United States, playing in two World Cups, before changing allegiance to Samoa in 2023.
"I have known Titi since my time coaching with USA Rugby and I have followed his progress closely throughout his career," Quins rugby director Billy Millard told the London club's website.
"He is a very experienced and dynamic tighthead who has achieved great success at club and international level and brings a wealth of knowledge and expertise to our front row."
