Late on Thursday, reports emerged that Chelsea were closing in on a player who they had almost signed during the final hours of the summer window, with Barkley preferring to wait on a move until he had recovered from a hamstring injury.
However, with the England international having now returned to training, Chelsea have pressed ahead with the transfer, which is said to be for a fee worth £15m plus add-ons.
The 24-year-old told the club's official website: "I'm overwhelmed, I'm looking forward to it and I'm really excited to get started.
"To be given a fresh start at a new club like Chelsea, it's unbelievable for me. I'm looking forward to continuing where I left off at the end of last season and hoping to improve and add more goals to my game."
Barkley leaves Everton having scored a total of 27 goals in 179 appearances in all competitions during a time where he has also won 22 caps for England.
The player will not be available to feature in Chelsea'a FA Cup tie with Norwich City on Saturday evening.