2009–10 Principality Premiership

The 2009–10 Principality Premiership was the fifteenth Principality Premiership season and the sixth under its current format. The season began in August 2009 and ended in May 2010. Fourteen teams played each other on a home and away basis, with teams earning four points for a win, and a bonus point for scoring four or more tries in a match. The losing team may also earn a bonus point if they lose by seven points or less.

British and Irish Cup qualification and play-off news
The top six clubs from the 2008-09 season qualified for the new British and Irish Cup competition. A play-off system was added at the end of the season for the top eight sides to qualify for the following season's British and Irish Cup. Only six teams can qualify for this from the eight. These play-offs do not decide the League Championship, as the league leaders at the end of the regular 26 game season will be Champions. The winners of the play-offs would win a one-off trophy.

The fourteen teams competing consisted of Aberavon RFC, Bedwas RFC, Cardiff RFC, Carmarthen Quins, Cross Keys RFC, Ebbw Vale RFC, Glamorgan Wanderers RFC, Llandovery RFC, Llanelli RFC, Neath RFC, Newport RFC, Pontypool RFC, Pontypridd RFC, Swansea RFC.

Elimination play-offs
Llandovery qualify for next season's British and Irish Cup, Aberavon are eliminated.

Llanelli qualify for next season's British and Irish Cup, Cardiff are eliminated.

Preliminary Semi-finals
'''Pontypridd were found guilty of fielding an ineligible player and so were expelled from the play-offs. Llandovery took their place in the semi-finals.'''

Final
Neath win 3–1 on try count