Fish's background saw him attend St Illtyd's Catholic High School in Cardiff and he joined Glamorgan Wanderers from his local club St Alban's.
He starred in the 2010 Six Nations campaign for Wales Under 20s and also during the IRB Junior World Cup Championship in Argentina in 2010.
Director of Rugby Phil Davies said: "We are all really pleased that Dan has signed new terms with us. He is a Cardiff boy who has been through our system and here for a number of years. He is one of the most talented footballers I have ever coached.
"It is a massive few years for him coming up and a massive opportunity for him to step up as one of the main candidates for the full back spot.
"It is a crucial time for him in his career and it's very important that he now delivers the performances that his talent suggests.
Fish added: "I'm happy to have signed a new contract and now it's a case of getting my head down and working hard to keep playing and stay in the team.
"I know how big an opportunity this is. I've been here for a few years and have had some good players in front of me for the No.15 jersey.
"Now I want to make the most of my chance, grab it with both hands and show the fans I am the person to fill that role long-term."