The Belgian goalkeeper signed for the Londoners in 2011 but only reported for club duty on Thursday - after his three-year loan stint at Atletico Madrid ended last season.
Courtois, 22, already has a wealth of experience under his belt, with this year alone seeing him win the La Liga title, play in a UEFA Champions League final and feature at the FIFA World Cup in Brazil.
And after making his long-awaited return to West London, Courtois said he felt as much a part of the team as anyone, largely thanks to the club's players.
"My new team-mates have been really great; they've welcomed me and you really feel that you are a part of this team," Courtois told Chelsea's website.
"Obviously it helped me that I knew some of the players before I arrived. When I signed here I had played for just one year in the Belgian league.
"I wasn't anybody (before that).
"In the last three years, I have been able to improve myself a lot and make my name in the football world.
"Now it is up to me to keep working hard here at Chelsea, keep improving myself and helping the team to achieve the things we all want."
Courtois was in the awkward situation of keeping against his employers in last season's Champions League semi-finals but he said he received a welcome reception from opposition fans as Atletico advanced past Chelsea over two legs.
"It was so nice to see that the Chelsea supporters appreciated my talent and my work at Atletico," he said.
"To see them clapping me was quite special and hopefully now I can give them joy by helping the team win games and trophies.
"That night was a really good experience but obviously I was playing for the other side at Stamford Bridge so it was a little bit weird.
"Now I'm just looking forward to playing a game at Stamford Bridge in the Chelsea colours."
Chelsea have two more pre-season friendlies - against Werder Bremen and Ferencvaros - ahead of their Premier League opener against promoted Burnley on August 18.