Onana has signed the paperwork on the contract, with United and Trabzonspor set to complete the final details within 24 hours.
Trabzonspor will pay Manchester United a maximum loan fee of £1.3m depending on performance-related add-ons next season, as well as covering the majority of Onana's wages, reports the Daily Mail.
There is no option in the deal for Trabzonspor to buy the goalkeeper next summer.
Onana moved to Turkey last September after it became clear he had no future at Old Trafford, with United telling him to find a new club following the arrival of Senne Lammens.
The 30-year-old Cameroon international impressed during his season at Trabzonspor, helping the club win the Turkish Cup in May.
It emerged last month that Trabzonspor wanted to keep Onana, with another season on loan seen as the most likely outcome.
A permanent move would have been preferable for United, allowing the club to recoup some of the £47.2million they paid Inter Milan for Onana in the summer of 2023.
Agreeing a new loan now avoids the awkward scenario of the former Ajax goalkeeper returning for pre-season training at Carrington later this month.
Onana, who still owns a house on the outskirts of Manchester, has two years remaining on his United contract.
United have decided not to reintegrate him to compete with Lammens, even though they will be in the market for a new back-up goalkeeper if Altay Bayindir leaves this summer.
The feeling at Old Trafford is that Onana's £120,000-a-week salary and extrovert personality would make it difficult to accommodate him on the bench.