“It’s a real honour to be signing with the Ospreys,” said Nayacalevu.
“There’s some world-class talent here and I’m looking forward to playing alongside them.
“Stepping into a team where quality experience is combined with exciting potential is ideal for me at this stage of my career.
“I believe I can contribute heavily to the squad this season, both on and off the field.”
Ospreys are 13th in the United Rugby Championship (URC) and have not given up hope of reaching the play-offs by finishing in the top eight, while they also face Scarlets in Europe’s Challenge Cup last 16 in early April.
Watkin suffered a serious knee injury playing for Wales against France in the Six Nations opener in January and will miss the rest of the season.
“It’s not often a player of Waisea’s calibre becomes available in the season,” said Ospreys head coach Mark Jones.
“He brings a wealth of experience and leadership credentials with him, which can only be a good thing for our youngsters coming through.
“We feel Waisea can make an instant impact and be a real difference for us in Owen’s absence.
“It stands us in good stead in a season where we still have a lot to play for.”
