Prieto told reporters on Thursday that he didn't use the watch for any illicit purpose and that it was on "airplane mode" during the game, a setting that shuts off any of the watch's communication abilities. The former big league pitcher is an extra coach who also serves as interpreter for the Diamondbacks.
Last month, the Boston Red Sox were fined by Major League Baseball for using an Apple Watch to relay signs to hitters.
Prieto said he knows having the phone during the game violated MLB's rules. He said he apologized to manager Torey Lovullo and volunteered to turn the phone over to MLB officials.
MLB spokesman Michael Teevan said the commissioner's office is looking into the matter.
"It was just an honest mistake," Lovullo said. "We asked, found out, and talked to Ariel, and that's what he told us. And in no way did it impact the game, and in no way is it going to impact the game tomorrow. It's just not something that we do or believe in. We're trying to steer our direction and steer our ship to tomorrow's game. We're aware of what's going on and we're cooperating. We're doing exactly what MLB is asking us to do."
The Diamondbacks beat the Rockies 11-8 on Wednesday night and Prieto was shown wearing the watch during the game's telecast. Arizona opens the National League Division Series on Friday against the Los Angeles Dodgers.
The watch was similar to ones worn by Red Sox personnel during the season. MLB revealed in September that the Red Sox had used the watches to steal signs from New York Yankees catchers during games between the teams.
The New York Post first reported the Diamondbacks news.
The Red Sox stopped the practice and cooperated with the league when they were alerted they were under investigation.
The Red Sox were fined an undisclosed amount, and commissioner Rob Manfred noted in his statement that "all 30 clubs have been notified that future violations of this type will be subject to more serious sanctions, including the possible loss of draft picks." MLB rules do not allow electronic devices to be used in the dugout.
Information from The Associated Press and ESPN's Bradford Doolittle was used in this report.