Kearney succeeds team-mate, Sean O'Brien, as the winner of northern Hemisphere rugby's ultimate individual accolade.
The 26-year-old scored six tries during the campaign and pushed himself into contention for the honour with a series of high-quality displays in the tournament's pool and knock-out stages.
Kearney scored two tries in the quarter-final victory over the Cardiff Blues, then followed that up by creating a try for Cian Healy and landing a superb drop goal in the thrilling semi-final success against Clermont Auvergne.
In last Saturday's final he played a key role in Leinster's record-breaking 42-14 win over provincial rivals Ulster at Twickenham.
Kearney was presented with his award, a specially struck gold medal, by ERC Chief Executive Derek McGrath at a ceremony in ERC offices in Dublin today.
"Rob was both rock-solid in defence and inspirational in attack during what was another magnificent Heineken Cup campaign for our 2012 champions Leinster," said McGrath.
"He scored six tries, ran powerfully, kicked superbly and the sheer all-round quality of his performances made him an excellent choice as ERC European Player of the Year for 2012."
Award winner Kearney admitted: "It really is an honour to have won this award. While I am delighted with the recognition, an individual player is powerless without the support of his team-mates.
"So I owe this to the Leinster squad, to the management and to all the backroom staff that support us."
The search for the best player from the 44 clubs in ERC's two tournaments, the Heineken Cup and the Amlin Challenge Cup, was undertaken by a judging panel which comprised of:
Stuart Barnes (Sky Sports), Raphael Ibanez (2011/12 Heineken Cup Ambassador), Stephen Jones (Rugby Correspondent, The Sunday Times), Emmanuel Massicard (Editor, Midi Olympique) and Gerry Thornley (Rugby Correspondent, The Irish Times).
The expert panel was presented with a longlist at the conclusion of the pool stages in both tournaments, and after the quarter-finals and semi-finals, that list was pared down to the following five players:
• Stephen Ferris (Back rower, Ulster)
• Rob Kearney (Full-back, Leinster)
• Ruan Pienaar (Scrum half, Ulster)
• Jonathan Sexton (Out-half, Leinster)
• Jonny Wilkinson (Out-half, Toulon)
The shortlist was also forwarded to the 44 teams, and their recommendations, as well as those of European club rugby supporters who voted on Twitter @ercrugby and on the tournaments' Facebook pages, were factored into the deliberations of the panel which reconvened in London, host city for the two European finals, to complete the selection process. The winner was then confirmed after the finals.
In 2010, ERC launched the highly-acclaimed ERC15 Awards, recognising the efforts of a number of outstanding contributors to the first decade and a half of European club rugby competition.
Two-time Heineken Cup winner Ronan O'Gara from Munster was the inaugural recipient of the ERC European Player honour as the best player over the first 15 years of the two tournaments.
Last year, Sean O'Brien from Leinster was voted ERC European Player of the Year for 2011.
ROB KEARNEY FACTFILE -
Born: March 26, 1986, Dundalk, County Louth
Height: 1.85m/6ft 1in
Weight: 95kg/14st 13lbs
School: Clongowes Wood College
Heineken Cup Appearances: 43
Ireland Caps: 38
British & Irish Lions Tests: 3 (v South Africa, 2009)