Johnson, 46, who notched his 300th win last season with the San Francisco Giants, told reporters he "wanted to go out on my terms.
"I definitely wanted to make the decision wholeheartedly and that I would stick to (retirement) ... I didn't want to make a hasty decision. I'm at peace with it," Johnson said.
With 22 years in the major leagues, the left-handed Johnson trails only Roger Clemens (six) in career Cy Young Awards. He ended last year 8-6 with a 4.88 ERA for the Giants, but suffered a tear in his rotator cuff and missed more than a month before returning in a relief role late in the season.
Johnson, at 6-feet-10 and sporting an intimidating scowl, won a World Series in 2001 with Arizona and played in eight Division Series and two League Championship Series. He used a blazing fastball from a unique sidearm delivery to record 37 shutouts and notch 4,875 strikeouts, which is second all-time to Nolan Ryan (5,714).