Seán Kevin O'Brien (born 14 February 1987) is an Irish retired professional rugby union player.

He did not feature for Leinster in the 2009 Heineken Cup Final, but evidence of O’Brien's emergence as one of the most exciting young back-row forwards in Europe came in the form of strong interest from France before Christmas of 2010.

The Irish Rugby Football Union moved quickly and O’Brien agreed a new three-year deal with Leinster the following January.

O'Brien started every game for Ireland during the 2011 Six Nations, winning the Man of the Match award against Italy.

O’Brien was ranked among the five best players at the 2011 World Cup, due in significant part to his standout performance against Australia where he led the team in both tackles and runs.

[13] In the final 2015 Rugby World Cup Pool D match, O'Brien punched Pascal Papé in the first minute of the win over France.

O'Brien was later cited by World Rugby, and received a one-game ban (reduced from the recommended minimum of two for mitigating factors including an "exemplary disciplinary record"), meaning he was unable to play in the quarter-final.

[17] O'Brien was named 2010–2011 ERC European Player of the Year following a season where Leinster were Heineken Cup champions.