The 36-year-old's last Town game was the 3-1 home defeat to Wrexham, which proved to be Paul Groves' last game at Blundell Park.
Groves signed the Scot on a year long deal earlier this season from Sheffield United after an initial loan spell. His time with Town saw 18 starts with just 3 goals, all headers.
The ex-Gillingham man leaves Town without impressing the majority of fans. He often seemed off the pace and a pale shadow of the man who reguarly terrorised Town's defence over the years.
He now moves onto Prenton Park and may even square up to his former team-mates on the final day of the season.
Rovers boss Brian Little was delighted at adding Onuora to his squad.
"Iffy’s experience and goalscoring record will be an asset to the Club as we enter the final stages of the season," he told the Tranmere website. "He is a strong character and will be a great addition to the dressing room. We have been looking for a big target man to lead the team from the front and Iffy fits the bill perfectly."