The long-serving centre has made a total of 122 appearances for Blues - scoring 13 tries - since making his debut against Edinburgh in January 2006.
A skilful and creative midfielder, Hewitt initially joined Cardiff RFC from Llantwit Major at the start of the 2004/05 season. He went on to play a total of 48 games for the Blue & Blacks.
Hewitt was part of the all-conquering Wales Under-21 Grand Slam winning team of 2005 and captained Wales Under-21s in the IRB World Cup in 2006.
The Cardiff-born back was also a key member of the Wales squad that won the Rugby World Cup Sevens in Dubai in 2009.
The 29-year-old was named vice-captain at Cardiff Blues in 2012 and hit a memorable milestone in 2013 when he made his 100th appearance for Blues when they played Edinburgh in the Guinness PRO12 at BT Sport Cardiff Arms Park.
However, Hewitt has been limited to just two appearances this season in all competitions and has now been forced to call time on his professional rugby career with a neck injury.
"It is hard to put into words how I am feeling at the reality of never wearing my Blues jersey again," said Hewitt.
"No rugby player expects to be told that they can never again play the game they love; it's the worst feeling. It was my childhood dream to play rugby for the Cardiff Blues; the team I followed as a boy and dreamt of representing. I have loved every moment of being a 'Cardiff Blue' and knew early on that it was a club I never wanted to leave.
"I'd like to thank everyone at the Blues; the coaching staff, the management, and the medical team for their invaluable support, especially during the past few months.
"I feel privileged to have played with so many great players and have forged friendships for life. I will miss everything about playing rugby and will always be a Cardiff Blue."
Richard Holland, Chief Executive at Cardiff Blues, said: "Dafydd has made a huge impact at our region over almost a decade and is a member of an exclusive club after making over a century of appearances since his debut in 2006.
"He has been an outstanding servant both on the field, including a spell as vice-captain, and off the field where he has been a true role model for young aspiring players to look to for inspiration and guidance.
"Dafydd is a hugely popular and much admired professional, from both his team-mates and our supporters, and he will be greatly missed as a player here.
"His passion for Cardiff Blues is evident to all and I know he will remain part of our region for many years to come. We wish him and his family every success for the future."
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