Romans (26) is still in negotiations to extend his Otago career but missed out on selection for the Highlanders next year.
He has taken the opportunity to find a job overseas and has been welcomed by Nottingham.
"Sean is a great signing for the club and brings with him a lot of experience from New Zealand rugby," Nottingham director of rugby Glenn Delaney said.
"We have been a little light at scrumhalf, with both Nathan [Jones] and Finlay [Barnham] carrying slight knocks through the first half of the season, meaning we have been a little exposed.
"We knew Tim Usasz wasn't coming back to the club after the World Cup, but agreed not to announce his departure until after that time. Tim has been a great servant to Nottingham and I would like to thank him for what he has helped us achieved here, and we all wish him well for the future.
"But we have to move on and we've been looking at who we could bring in, and I am delighted with Sean's signing.
"Sean's signing provides us with greater strength in depth and a healthy competition for places in the scrumhalf position."
Romans made 11 appearances for the Highlanders, played for the Junior All Blacks in 2009 and has been a regular in the Otago squad in recent years.
He is awaiting clearance of his visa application, but is expected to join Nottingham as soon as possible. His team-mates will include former Highlanders and Otago lock Filipo Levi, and Japan and former Highlanders first five James Arlidge.
