Springbok World Cup winner Nkosi quits Bulls after troubled season
A Bulls statement said they and Nkosi had mutually agreed to terminate his contract one year before it was due to expire.
After moving from Durban outfit Sharks, Nkosi disappeared from the Bulls camp last December, triggering a nationwide search.
He was found at the home of his father in eMalahleni, a mining town 105 kilometres (65 miles) east of Pretoria.
The player admitted he had mental health challenges and, after treatment, scored three tries in four matches for the Bulls before falling out of favour.
Bulls rugby director Jake White said recently that a change of environment might be best for Nkosi, and he has been linked with the Cape Town-based Stormers.
His 16 Test appearances included deputising for injured Cheslin Kolbe in a World Cup semi-final victory over Wales in Japan four years ago.