The company, formed in 1860, is the largest building society in Wales and the 12th largest in the UK. It has 51 branches across the country, many based in north and mid-Wales where a majority of the Welsh Premier League teams are based.
John Deakin, Welsh Premier League secretary, said: "It was important to us to seek sponsorship from within Wales, and Principality has enabled us to do that."
Principality is also sponsor of the Welsh Rugby Union Principality Building Society Premiership and of the Olympic swimmer David Davies.
Peter Griffiths, Principality Building Society chief executive, said: "Football is up there as our 'other' national sport and its popularity is on the increase across the nation."
The Principality Building Society Welsh Football Premier League kicks off on August 18.
Dominic White, head of marketing at Principality, said: "Our sponsorship of both the Welsh Football Premier League and the rugby Premiership allows us to build a strong presence throughout the whole of Wales. We aim to be associated in people's mind with the best of Welsh sport."
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