Leinster wing cum centre Horgan won 65 caps for Ireland, made over 200 appearances for Leinster and featured in all three Lions Tests in New Zealand seven years ago.
"Every professional sportsman realises that at some point their career will come to an end. I have been extremely fortunate to have played regularly for my province and with Ireland since my debut with Leinster in 1998 and
"While I am disappointed that injury has forced me to retire now, I know I have been very lucky to have had such a long and largely injury-free career," said Horgan, who wore Lions colours eight times, although he only made one start.
"It has been a tremendous honour to represent my province and my country. When you first start playing sport that is the ultimate ambition but few people are lucky enough to realise it. I am just so grateful that I was given that opportunity.
"I am equally fortunate to have played at a time when Leinster and Ireland experienced such great success and alongside a group of players with unprecedented talent and ambition."
Horgan has been unable to train properly for the past two seasons and hasn't played at all this term, with his last Leinster appearance coming in the then Magners League Grand Final defeat to Munster back in May of 2011.
He had initially battle back from a knee problem he had aggravated in January of that year to play a key role in his province's second Heineken Cup success and became the first Leinsterman to make a double century of appearances in the process.
But he missed out on selection for Ireland's 2011 World Cup squad in New Zealand having not represented his country since November 2009 and has now had to finally face defeat in his battle to overcome persistent knee issues.