The Spain winger turned the club down last summer in order to go on loan to bitter city rivals Real Madrid, and admitted he could be a target for some supporters.
"I am a little bit worried by the fans' reaction, but I will have to prove my worth on the pitch, scoring goals, and by helping them to be successful," Reyes said.
Reyes said it had been his dream to join Atletico for 10 years after completing his move to the Liga side.
The 23-year-old joined on a four-year deal for an undisclosed fee after spending last season on the other side of the Spanish capital.
The former Sevilla man said Atletico had registered their interest in him when he was just 13, and that he had wanted to play for them ever since.
"I am happy I am back in Spain again and with the club I wanted to join. The club have been interested in me a long time and tried to sign me from Sevilla when I was 13. I owe it to them," Reyes said.
"Real wanted me right up until the last moment but Atletico made a great effort to get me."
Reyes was presented to the media in a no-frills morning press conference, before joining the squad as they travelled to the Netherlands for a pre-season tournament.
Atletico have been hoarding attacking midfielders this summer, signing Reyes along with Benfica's Simao Sabrosa and Liverpool man Luis Garcia.
However, they have sold Martin Petrov to Manchester City and skipper Fernando Torres to Liverpool.