The 35-year old rejoins the Ospreys where he previously had six years involvement in a number of roles.
He got his first coaching opportunity at Glamorgan Wanderers, where after a long association with the club as a player, he was Head Coach for three and half years, securing promotion to the Premiership during his time in charge.
He began working with the Ospreys in 2005 when as WRU Performance Skills Coach he played a key role in devising, implementing and developing programmes in the newly formed academies across Wales.
For the next five years, while still WRU contracted, his regional work was focused entirely on the Ospreys, initially on the development pathway helping to bring through a host of players such as Justin Tipuric, Ryan Bevington, Joe Rees, James King, Rhys Webb, Dan Biggar, Ashley Beck, Matthew Morgan, Eli Walker and Tom Prydie.
In 2008, his role was extended to encompass the post of Skills Coach to the senior squad as well as the monitoring and development of individual performance.
Rees said: "This is a really exciting opportunity for me and I can't wait to get stuck into it. I realise that working for a region with such high expectations, both internally and externally, comes with real pressure but I believe that along with Steve Tandy and Jonathan Humphreys I can help to move the Ospreys forward."
A highly rated coach, away from the Ospreys he assisted Rowland Phillips at Neath between 2006 and 2008 as the All Blacks claimed successive Premiership titles and acted as a consultant to new Ospreys Head Coach, Steve Tandy, at Bridgend Ravens.
In addition, his talent identification and development expertise has seen him working with Wales at several age-grade levels, and last season he was Head Coach of Wales U18's, with the highlight of his time in charge being a significant away win in France at the end of the campaign.
Rees has also worked with Wales Sevens, before spending the last eight months working in Italy with PRO12 outfit Aironi on a secondment basis, where he was responsible forimplementing an academy structure and effective succession planning / talent identification structures for an organisation just a year old at the time of appointment.
Rees continued: "I've been away from the region for a short while and the experience I've gained in Italy will prove invaluable, allowing me to understand other rugby cultures and to look at a different way of working. With the experiences I've got at international level and my knowledge of the group of players we'll be working with, I'm optimistic about what we can achieve here."
Andrew Hore, Ospreys Chief Operations Officer, added: "Gruff is a young coach who has already gained enormous experience and an outstanding reputation in the game. He is renowned for his talent ID and development work, areas in which his record speaks for itself, and he is someone with the ability to impart knowledge "on the shop floor", so to speak."
Joe Lydon, WRU Head of Rugby, said: "Following discussions with the Ospreys and WRU Performance Skills Coach Gruff Rees, the WRU are happy to support both parties in releasing Gruff from his current regional and national age-grade coaching roles to take up the position of Backs Coach within the Ospreys senior squad.??
"Having recently returned from a coaching secondment with the Italian club Aironi, the WRU believe this to be a fantastic opportunity both for Gruff to further develop as a Welsh coach with the Ospreys newly defined coaching team, and the players within the Ospreys region. All within the WRU wish Gruff all the very best in his new key regional role."