The 2009–10 Leinster Rugby season was Leinster's ninth competing in the Celtic League which they finished runners up in, alongside which they were competing in the 2009–10 Heineken Cup as defending champions until they were knocked out at the semi-finals by Stade Toulousain who went on to win the competition.

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

Players may hold more than one non-World Rugby nationality.