The 24-year-old deleted his Twitter account last week after screen shots of comments he purportedly made appeared on the website of Miami New Times. Cosart denied betting on baseball.
"I have never, nor would I ever, bet on the great game of baseball," Cosart said in a statement Friday. "Major League Baseball conducted a thorough investigation, and I cooperated fully with them and their investigators throughout that process. I'm sorry for any distractions this may have caused the Marlins, my teammates, coaches, and our incredible fans. I'm glad to bring closure to this situation before Opening Day and I look forward to a great season."
The commissioner's office said Friday its inquiry "did not reveal any evidence to suggest that Cosart, who fully cooperated with the investigation, bet on baseball."
MLB said Cosart was fined for violating a rule that "prohibits players from placing bets with illegal bookmakers or agents for illegal bookmakers. This rule is strictly enforced and applies to gambling with illegal bookmakers on any sport or event."
The Associated Press contributed to this report.