World Superbike champion in his rookie year in the series in 2009, Spies has failed to translate that form across to MotoGP.
Riding for the Tech3 Yamaha team in 2010, the 28-year-old scored two podium finishes en route to finishing sixth in the championship, and so earning a factory ride for 2011.
Although overshadowed by his more experienced team-mate in 2010 champion Jorge Lorenzo, who was second last year, Spies at least managed a victory and three other podiums in finishing fifth.
This year, however, has been a disaster for the American who is currently languishing in 10th in the standings without a top- three finish to his name.
That is in stark contrast to Lorenzo who leads the championship after chalking up five wins and three seconds.
Via his Twitter account, Spies posted a link to website SuperbikePlanet.com on which he wrote: "On Monday I made a comment on Twitter to cause a stir.
"I'll be more in saying I have chosen to leave Yamaha after this season for a litany of reasons.
"I will explain more when I feel it is appropriate. I'm discussing my situation with my sponsors and they are behind me.
"There are a few people at Yamaha who I hate to leave. And they know who they are.
"I wish Yamaha the best. I won't make any further comments until the offical announcement comes out."
Via a further tweet, Spies added: "I'm extremely happy to push for the rest of the season.
"To my fans, and myself, next year starts a new chapter. And yes I'm pumped."