The 30-year-old will officially link up with the Falcons later this summer subject to a work permit.
Nagusa, who spent last season with French club Aurillac, boasts a wealth of international experience - he has represented Fiji 16 times - while he has also captained their Sevens team on numerous occasions.
Newcastle director of rugby Dean Richards is excited by the prospect of Nagusa playing at Kingston Park as he believes the forward will add the X-factor to his side.
"Nemani is an exciting player with pace, power and handling ability," Richards told Newcastle's website. "He is part of a strong Fiji side who are making big strides on the international scene. He has that X-factor about him and we look forward to his arrival once the work permit process has been completed."
Nagusa becomes Newcastle's seventh signing of the off-season after Logovi'i Mulipola, George McGuigan, Tom Arscott, Johnny Williams, Guy Graham and Connor Collett.