Carney Chukwuemeka rejected a massive contract offer from Aston Villa to leave for Chelsea.
The west Midlands clubs were desperate to hold onto the midfielder and offered him a new contract worth £100,000-per-week (£5.2million-a-year), says the London Evening Standard.
Had he accepted, Chukwuemeka, 18, would have been one of the highest-paid youngsters in the world.
Speaking to Chelsea's in-house media team following his arrival, he explained his decision to join the west Londoners.
"It's been quite hectic but I couldn't get Chelsea out of my head the last few days so I'm just happy and relieved that it's all done," he said. "I'm so excited and just can't wait to get on the pitch, meet all the players and try to win games and trophies with Chelsea."