Three-teams from the competition's current 18-team format are to be cut, with one Australian side and two South African franchises poised to be removed.
Japanese side Sunwolves will move to the Australian conference, having spent their brief time in the competition to this point in the Africa 1 conference.
All five New Zealand-based teams will keep their places, with the franchises who will lose their spot due to be informed by their national unions in due course.
Eight teams will contest the finals series, with the three conference winners and five wildcard teams progressing to the knock-outs.
The announcement came soon after Western Force beat Southern Kings 46-41 in a game contested by two of the sides whose future appears to be uncertain.
SANZAAR Chairman Brent Impey said in a statement: "The decision to revert to a 15-team format reflects a consensus view of the mandated SANZAAR Executive Committee that met in London recently.
"It was not the determination of any one Union or stakeholder and follows a thorough assessment and review of the tournament over the last nine months.
"This decision has not been an easy one and we recognise the difficulty associated with reducing the number of teams in Australia and South Africa.
"Naturally we understand that there will be some very disappointed franchises but the tournament's long-term future and the economic reality of the business at present is something that had to be addressed.
"The decision to retain the Sunwolves is linked directly to SANZAAR's strategic plan for the future. The potential for growth of the sport in Asia off the back of the establishment of the Sunwolves and the impending RWC in 2019 is significant.
"It remains an obvious focus for the organisation and a Japanese Super Rugby franchise is key to that strategy."