The member-owned organisation first lent its Welsh brand to the domestic league’s top division in 2005, and is now set to complete 14-seasons directly supporting the national game’s semi-professional league, before standing down as title sponsor of the competition.
In addition, Principality will also retain title sponsorship of the WRU National Youth Leagues for 2018/19 – to be known as the Principality Youth Leagues.
Julie-Ann Haines, Chief Customer Officer at the Principality Building Society, said:
"We are delighted to extend our successful sponsorship of the WRU Premiership for one more season. The Premiership is something that is very close to our hearts as an organisation and we know how much local rugby means to our Members and the communities they live in.
"We wanted to make sure all the clubs had our total support in the final year of the current format of the league. It is fantastic news for the clubs, the WRU and ourselves as proud sponsors of a league that means so much to us and our Members.
"As sponsors we are fully supportive of the great work the WRU are doing to try and make club rugby as competitive as possible together with the new format for 2019/2020, which will find the best club side in Wales.
"Our sponsorship next season will also mean we will support the National Youth Leagues for one more season. We’ve had a great reaction from the local clubs for the support we have given them this year and it has been a pleasure to bring their rugby communities together. We’ve travelled to clubs across the whole of Wales to meet the youth teams and via social media have helped raise the profile of the teams and their communities."
WRU chief executive Martyn Phillips said: "Principality Building Society’s continued commitment is welcomed by all in Welsh rugby.
"The Principality Premiership is entering the third and final season of ring-fencing and from 2019/2020 will be reduced to 12 (from 16) teams.
"This gives a final year of absolute stability for all of the Principality Premiership clubs next season, which will prepare the way for the jeopardy of promotion and relegation the year after and Principality’s support increases that stability.
"Our long term partnership with Principality, which includes support for Principality Stadium, plays a vital role in helping us safeguard the future of our national game.
"And their direct involvement in the grass roots and community end of the game, alongside their continued sponsorship of the Principality Youth Leagues, means their overall contribution to Welsh rugby is a hugely significant."
Bridgend Athletic face Pontypridd at Rodney Parade (7pm) on Saturday to determine the winner of the 2017/18 Principality Youth Leagues and Merthyr) were crowned champions of the Principality Premiership – for the second successive season – last weekend.