"Aseli is one of the most exciting backs in world rugby, and we are delighted that he has signed for us," said Harlequins rugby director Conor O'Shea.
"He comes from a winning culture at the Chiefs, is equally at home on the wing or at centre, and I am sure will fit seamlessly into the way we play here and add to the quality we already have."
Tikoirotuma, 27, will join Harlequins in August when his contract with Waikato-based New Zealand team, the Chiefs, expires.
He has played a key role in the Chiefs' Super Rugby successes in the past two seasons, scoring 11 tries in 33 games, and made his debut for Fiji last year.