Correa admitted that he tapped into the Astros' player scouting database and emails from 2013 to at least 2014 to use the information to help the Cardinals draft players.
Correa pleaded guilty to five counts of unauthorized access of a protected computer. The 35-year-old faces up to five years in prison in each charge. Sentencing is scheduled for April 11.
"I accept responsibility in this case," Correa told U.S. District Judge Lynn Hughes. "I trespassed repeatedly."
Correa was fired last summer by the Cardinals after he was promoted to director of baseball development a year earlier.
U.S. Attorney Kenneth Magidson estimated the hacking cost the Astros about $1.7 million.
"It has to do with the talent that was on the record that they were able to have access to, that they wouldn't have otherwise had access to," Magidson said.
The Cardinals face possible discipline from Major League Baseball that could include lost draft picks. Team chiarman Bill DeWitt Jr. declined comment after the decision.
Astros general counsel Giles Kibbe called it a difficult day for everyone in baseball and said all information from the case would be given to the commissioner's office.