Ospreys, formed in 2003, regularly played at St Helen’s before moving into their home at Swansea City’s 21,000-seat stadium in 2005.
Chief executive Lance Bradley announced in January the region would be leaving for a smaller venue.
Ospreys had considered a permanent move to Bridgend after staging both a league and European fixture at the Brewery Field in the 2023-24 season, but opted to remain in Swansea.
Ospreys say the move will involve a “multi-million pound investment” in St Helen’s, including a 4G pitch and new stands.
The move will require significant redevelopment of the famous old ground, which is still the home of Swansea RFC, to initially increase the current 4,500 capacity to around 8,000.
Ospreys are finalising the proposed layout of the site before submitting designs for planning approval.
Phase one of the proposed development would see the installation of a new and relocated pitch, refurbishment of the clubhouse and terrace, plus the installation of new stands.
There would also be a fan zone designed to provide entertainment before and after matches.
Phase two would see the installation of a state-of-the-art training facility, including gym and office space, plus a training barn, which will also be available for community use.
