The 30-year-old is currently in his fourth season with Agen, but the French club have agreed to release him from his contract early.
Ratuniyarawa is keen to further his career in England and will bid to secure a starting spot at Franklin’s Gardens.
Saints have been on the look-out for a new second row forward since the end of last season and Christian Day’s bicep injury has added to their need for reinforcements.
And Ratuniyarawa will bring plenty of experience, having played 69 times for Agen and 19 times for Fiji, who he represented during the 2015 World Cup.
He also helped his country win this year’s Pacific Nations Cup and links up with Fiji team-mate Campese Ma’afu at the Gardens.
“I’ve enjoyed playing for Agen, which is a great club,” Ratuniyarawa said. “But now I’m looking forward to the next part of my career, in England.
“It is a decision which I have made as much for my family as for myself, and I am grateful to Northampton’s coaches for giving me the opportunity.
“I know from my Fiji team mate Campese Ma’afu that it is a great club and hopefully I can help the Saints achieve success in the Aviva Premiership.”
Ratuniyarawa stands at 6ft 6in tall and weighs in at 112kgs.
And Saints boss Jim Mallinder believes he has the ability to be a big hit in the Aviva Premiership.
“Api is a strong, athletic, physical lock and he has the potential to make a big impact in the Aviva Premiership,” Mallinder said.
“He has plenty of experience, both at international level and in the challenging environment of French club rugby, and we’re looking forward to seeing what he can do at Franklin’s Gardens.”
Agen’s chairman of the board Alain Tingaud says that Ratuniyarawa leaves with the club’s best wishes.
“Api has been a good servant to Agen for the last three seasons and he has been a big part of our team,” Tingaud said.
“We understand that it is his family’s desire to live in an English-speaking country, which is why we have agreed to release him early from his contract, and Api leaves with our best wishes for success in England with Northampton.”