O'Brien was presented with his award, a specially struck gold medal, by ERC chief executive Derek McGrath at a ceremony at the player's home ground, the RDS.
"Sean was an inspirational character at the heart of the Leinster pack in their victorious Heineken Cup season and proved himself to be a major force both as a ball carrier and defender," said McGrath.
"He played across the back row, scored vital tries and picked up three Heineken Cup man-of-the-match awards.
"His power, athleticism and consistency over the course of the Heineken Cup made him an excellent choice for the ERC European Player of the Year award."
In 2010 ERC, the governing body of Europe's two club rugby tournaments, launched the highly acclaimed ERC15 Awards, recognising the efforts of a number of outstanding contributors to the first decade and a half of European competition.
The inaugural recipient of theERC European Player award, as the best player over the first 15 years of these tournaments, was Munster's Ronan O'Gara.
For the 2010/11 season, ERC opted to present one award to recognise the best player of the season from both of its tournaments - the Heineken Cup and Amlin Challenge Cup - selected by a panel of five experts.
The selection panel comprised three members of last year's ERC15 Awards committee, Stuart Barnes (Sky Sports), Jean-Roger Delsaud (Midi Olympique) and Stephen Jones (Sunday Times). They were joined by Heineken Cup Ambassador, Raphael Ibanez, and Gerry Thornley (Irish Times).
After studying the performances of players from the 24 Heineken Cup and 20 Amlin Challenge Cup teams in this season's campaign, the selection panel presented a five-man shortlist to ERC:
• Jamie Heaslip (Leinster)
• Isa Nacewa (Leinster)
• Sean O'Brien (Leinster)
• Sergio Parisse (Stade Francais)
• Soane Tonga'uiha (Northampton Saints)
The shortlist was also presented to the 44 competing teams and their recommendations were factored into the deliberations of the selection panel which reconvened in Cardiff, host city for the two European finals, to complete the selection process.
The winner was confirmed after all five shortlisted candidates had been seen in action in the two finals.
"It's been a fantastic year for me, and more importantly for Leinster. It's great to be presented with an award like this, but it has more to do with the team than it has to do with me," said O'Brien.
"Hopefully, this is only the start rather than the end of our success in Europe, and of course, we've got another trophy to aim at this weekend."
Born: February 14, 1987, Ardistan, County Carlow
Height: 1.88 m (6ft 2in)
Weight: 108 kg (16st 13lb)
School: Tullow Community School
Clubs: UCD, Tullow
Ireland Caps: 9 (W7, L2)
Test Debut: Ireland v Fiji at the RDS, November 21, 2009
Sean won caps for Ireland at Under-18, Youths, Under-20 and Under-21 levels, including a Six Nations Grand Slam with the Under-20s in 2007, and has now made 55 appearances (scoring 55 points) for Leinster. He has nine Irish caps to his credit to date.
SEAN O'BRIEN IN EUROPE
P18 W14 D1 L3
Sean won his second Heineken Cup winner's medal in Cardiff last weekend having been an unused replacement in the 2009 final triumph over Leicester Tigers. He made his Heineken Cup debut for Leinster against Castres Olympique in December 2008 and has now played 18 times in Europe.
2010/11 Heineken Cup Appearances -
09/10/2010 - Round 1: Leinster 38 Racing Metro 92 22 (number 6 *) try
16/10/2010 - Round 2: Saracens 23 Leinster 25 (number 6)
12/12/2010 - Round 3: Clermont Auvergne 20 Leinster 13 (number 6)
18/12/2010 - Round 4: Leinster 24 Clermont Auvergne 8 (number 6) try
15/01/2011 - Round 5: Leinster 43 Saracens 20 (number 8 *) try
21/01/2011 - Round 6: Racing Metro 92 11 Leinster 36 (number 8 *) try
09/04/2011 - Quarter-final: Leinster 17 Leicester Tigers 10 (number 7)
30/04/2011 - Semi-final: Leinster 32 Toulouse 23 (number 7)
21/05/2011 - Final: Leinster 33 Northampton Saints 22 (number 7)
* Heineken Cup man-of-the-match