Calvin Wellington has moved to the Dragons who have paid a transfer fee.
The centre has returned to Rugby Union after spending three years with the Saints.
Calvin, 21, made his debut for Saints at the Magic Weekend in 2016 – his only appearance in the Red Vee.
He also played one game for Sheffield on loan in 2017.
Saints CEO Mike Rush said: “Calvin has been a popular member of the club since he initially joined our Academy and we are sad to see him leave.
“Calvin had agreed to stay with the club in 2018 but the lure back to rugby union and his home was too strong. We wish him all the best for the future.”
The centre signed professional terms with the club after originally impressing on a two-week trial.
He caught the eye of Saints’ scouts playing Rugby Union for Aberavon Quins – but he also played League for Wales at 16s and 18s – as well as the South Wales Scorpions.