The 30-year-old joined Northampton at the start of last season and will now move across the midlands to join Warriors until at least 2012.
Best said: "I'm very excited to have signed for Warriors. It's going to be a new challenge for me and a big move for me as I join another English club.
"The development of the club is clear to everyone. From the coaching staff to the supporters, even the stadium development and infrastructure, is right. Every aspect is set up right.
"I went to meet (Worcester director of rugby) Mike Ruddock, I knew about him as a coach and his ethos, and his passion and ambition for success can only rub off on you.
"Everyone is singing from the same hymn sheet and keen for success and that makes you want to play for the club."
The Belfast man admitted that getting back into the Ireland senior side remains a goal, with his last cap against Argentina during the 2007 Rugby World Cup.
"Like everybody who has played international rugby, if someone phoned me up from Ireland and asked me to play I would be there in a shot," he added.
"My priority, though, is getting my game right for Worcester at club level. You need club success if you want to do well internationally, it is that simple.
"I will leave Northampton at the end of the season with fond memories. I have lots of friends at the club and got on well with the coaching staff. I'm a professional and will do absolutely everything to make sure the club is successful as we can be this season.
"After that I have a new challenge at Worcester and I'm looking forward to it and getting started."
Commenting on Worcester's latest signing, Mike Ruddock said: "I'm delighted that Neil Best has decided to join Warriors next season. This is a great signing for Worcester Warriors and we look forward to welcoming him to Sixways.
"Neil is a highly respected figure in British and Irish rugby and brings with him a wealth of experience at international level and a no nonsense approach combined with intelligence and skill.
"He is just the sort of player we need in our squad and I know he will add leadership to our team and will always lead from the front."