The former hooker, who was capped eight times by Ireland, has also had a considerable impact off the playing field. Following his retirement due to injury, Clarke moved into coaching and established the Ulster Rugby Academy.
Clarke was then appointed as Assistant Coach to Mark McCall in 2004 and he helped guide Ulster to the Celtic League title in 2006. The following season Clarke took up the position of High Performance Manager with the IRFU where he was responsible for overseeing the national age-grade and development programme. Five years later Clarke returned to Ulster as Elite Performance Manager before being appointed as Ulster's Assistant Coach ahead of the 2014/15 season.
Commenting on the announcement, Clarke said:
"It has been a great privilege for me to represent Ulster both as a player and as part of the coaching team. When I was given the opportunity to move back here in 1998 and represent my home province, it was an easy decision for me. As a proud Ulster man, to share in some of the club's greatest days, has been a great honour.
"The Ospreys are a club that I am very familiar with and I have been very impressed with the standards they set and the ambition they have, so I am really excited about the next chapter in my career and the challenges it will bring.
"In the meantime, we still have plenty of work to do at Ulster so I am looking forward to, and fully focussed on, the remainder of the season as we strive towards securing a spot in the PRO12 playoffs."
Ulster's Director of Rugby, Les Kiss, added:
"Allen has made a great contribution to Ulster Rugby over the past twenty years. His attention to detail, work ethic and his intricate knowledge of forward play has brought significant value to our programme here.
"Away from the senior team, his work in establishing the Ulster Rugby Academy has been invaluable and I'm sure we will see the rewards from that for years to come.
"We wish Allen the very best for his new role and we know over the next couple of months he will give everything to ensure we finish the season on a high."