The 2008–09 Leinster Rugby season was Leinster's eighth competing in the Celtic League alongside which they competed in the 2008–09 Heineken Cup.

Though Leinster were ultimately to lose their Celtic League crown to rivals Munster, they were to cap a great year for Irish rugby with their victory over the Leicester Tigers in the Heineken Cup Final at Murrayfield.

[1] Note: Flags indicate national union under World Rugby eligibility rules.

Leinster's average Celtic League attendance was 14,728, up 415 on the 2007-08 season.

[2] Leinster's Heineken Cup semi final against Munster, held in Croke Park set the record for the highest attendance in club rugby history at over 82,000.