Debuting for Auckland in 2012, Fox-Matamua returned to the provincial squad in 2014 after a year-long spell with North Harbour, where he also played Sevens rugby.
The 23-year-old will arrive at Connacht for the beginning of the pre-season period in June.
Commenting on the new signing, Connacht Head Coach Pat Lam said:
"We're delighted to be in a position to announce Nepia. With Willie (Faloon) returning to Ulster, another quality openside flanker was needed for next season's squad.
"I have worked with Nepia at Auckland. He is a great team man and having captained different sides in his young career, he also possesses excellent leadership qualities. Technically he is an out-and-out number seven with natural ball skills that have seen him excel in Sevens rugby as well. I know that he is the type of player and person that will fit in very well to the Connacht community."
CEO Willie Ruane added:
"In addition to the high number of current players committing their futures to Connacht, it's great to be adding to the quality and depth of the squad with a player like Nepia. We are confident he will bring a lot to our environment as we bid for honours in both the PRO12 and European competitions."
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