The 2018–19 Principality Premiership was the third season of the new format of the Principality Premiership, the top tier of club rugby in Wales run by the Welsh Rugby Union. It was contested by sixteen Welsh clubs following an expansion from twelve teams at the start of the 2016–17 season. The 2018–19 season saw the end of "ring-fencing" and relegation returned for this season with four teams being automatically relegated to the 2019–20 WRU Championship and a fifth team avoiding relegation by winning a play-off. The competition was won by Merthyr for the third consecutive year.