Arranged through the Chelsea foundation, Community Day gave fans the chance to watch the first-team train before meeting players and coaches afterwards for pictures and autographs.
Spain international Costa, who scored in Chelsea’s 2-0 win over Leicester on Saturday, was one of the last players to leave Cobham after staying behind to meet and greet supporters.
“This was the first Community Day I have been involved in and it was a pleasure to play my part,” the Brazilian-born striker said.
“It’s really nice to see the smiles on the faces of the fans. It is important the club shows a commitment to the fans and as players we are delighted to do our part.
“I’m sure they had a nice morning and hope it has helped in some way but it was also good for me to join in, everybody’s had a great morning.”
Chelsea supporter Luke Perver, 17, who suffers from Cerebral Palsy, was taken to Cobham as a surprise by his mother Sally.
“I didn’t tell him until we went to leave this morning and when I did his face absolutely lit up!” said Sally.
“It is out of this world for him to meet all the players and we are both so thankful.
“He enjoyed meeting John Terry and Petr Cech, but also the new players like Diego and Cesc Fabregas.”
It was also a memorable day for Chelsea supporters Mark Cunliffe and wife Simone. The pair, who were at the 2012 Champions League Final, met club legend Didier Drogba, who scored the winning penalty for Chelsea in Munich.
“To see Drogba back here again is so exciting,” said Simone. “We thought that was the end of him in a Chelsea shirt when we left Munich so to have him here again is fantastic.
“The new players have been amazing so far this season so to meet them in the flesh so soon in to their Chelsea careers is brilliant."