The former Wales A lock, nicknamed 'Bull', has been watched by Gloucester's head coach Dean Ryan this season and told www.gloucesterrugby.co.uk "I am 29-years-old and it's a great move for me - what can I say - I am delighted.
"This is a fantastic opportunity to have a crack at Premiership rugby and to be joining a club of Gloucester's calibre is a great feeling.
"I have experience of European Cup rugby with Pontypridd and have felt things have been going pretty well for me over the last two years.
"I enjoy the physical side of things and hopefully my performances will reflect that. It is just a fantastic opportunity for me and I am really thankful for the chance."
With 15 appearances for the Pirates this season James is an integral part of their side and Gloucester's head coach Dean Ryan could not hide his delight at his capture.
"We feel Will is a player who has slipped through the net at Premiership level but we have monitored his progress for sometime," Ryan said.
"He has a lot to offer, has Heineken Cup experience and can bring us that physicality in the second row we are keen on.
"He is a big, physical player and will add a lot to the second row options we already have available at the club. It really is good news for us."