The young centre made seven European appearances with Gloucester last season, Allen won a European Challenge Cup medal when London Irish were defeated in a memorable final. Allen has two Heineken Cup caps under his belt so far this season, including the Round 2 match against French side Agen when he recorded his first two Heineken Cup tries.
"Since I became a professional with Gloucester, everything has happened so fast I cannot really believe it," Allen told www.gloucesterrugby.co.uk
"My initial ambition was to try and break into the first team and I am just delighted to have been able to contribute something to what we are trying to build at the club.
Gloucester's head coach Ryan, who has already sealed new deals for centre Jack Adams and prop Jack Forster, was equally as thrilled for the centre.
"Anthony's quality is obvious to see and to have the chance to work with him over the long-term is a fantastic position for our club to be in.
"He is a tremendous talent, makes very few mistakes and is an individual we would aim to build a team around in the future. We were delighted when he made his England debut because it confirms our thoughts on him are correct and I am sure he will only get better."