Ream, 24, agreed to personal contract terms with Bolton, earned United Kingdom visa approval and passed a physical. He played for New York from 2010-12, made his U.S. debut in November of 2010 and has gone on to earn six more caps.
"This was a hard decision for me considering the fact that I enjoyed playing for the Red Bulls and was looking forward to the upcoming MLS season. However, after consulting with those closest to me, I decided that this was a once in a lifetime chance that I could not pass up," Ream said.
Ream started 28 games last year, only missing time due to international games. Ream was a finalist for MLS Rookie of the Year in 2010 and was an MLS All-Star last season. He was selected in the second round of the 2010 MLS SuperDraft.
"This was a difficult decision for us as a club, but at the end of the day, we could not turn down this opportunity to help Tim's development as a player," said New York general manager and sporting director Erik Soler.
"He has become a mature professional during his time here and we will fully support him as he moves to Bolton."
Bolton is 17th in the Premiership, one point above the relegation zone.