2009–10 Celtic League
The 2009–10 Celtic League was the ninth season of the league now known as Pro12 and the fourth with Magners as title sponsor. The season began in September 2009 and ended with the Grand Final on 29 May 2010. Ten teams played each other on a home-and-away basis, with teams earning four points for a win, two points for a draw 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.
The ten teams competing consisted of the four Irish provinces, Munster, Leinster, Connacht and Ulster; two Scottish regions, Edinburgh Rugby and Glasgow Warriors; and four Welsh regions, Cardiff Blues, Newport Gwent Dragons, Ospreys and Scarlets.
This season saw the introduction of a play-off structure similar to the English Premiership in order to determine the overall winner. The Ospreys won the final, defeating Leinster 17–12 in Dublin.
Teams
| Team | Coach | Captain | Stadium | Capacity |
 Cardiff Blues | Dai Young |  Paul Tito | Cardiff City Stadium | 26,828 |
 Connacht | Michael Bradley | John Muldoon | Galway Sportsgrounds | 7,000 |
 Edinburgh | Rob Moffat | Mike Blair | Murrayfield Stadium | 67,144 |
| Glasgow Warriors | Sean Lineen | Alastair Kellock | Firhill Stadium | 10,887 |
| Leinster |  Michael Cheika | Leo Cullen | RDS Arena | 18,500 |
| Munster | Tony McGahan | Paul O'Connell | Thomond Park Musgrave Park | 26,500 8,300 |
| Newport Gwent Dragons | Paul Turner | Tom Willis | Rodney Parade | 11,700 |
| Ospreys | Scott Johnson | Ryan Jones | Liberty Stadium | 20,520 |
| Scarlets | Nigel Davies | Mark Jones | Parc y Scarlets | 14,870 |
| Ulster | Brian McLaughlin | Rory Best | Ravenhill | 12,125 |