Derek Jeter -- I'm Retiring ... After One Last Season

Derek Jeter just announced ... he's retiring after one final season with the New York Yankees.

Jeter just posted his explanation on Facebook ... saying, "I could not be more sure. I know it in my heart. The 2014 Season will be my last year playing professional baseball."

He continues, "For the last 20 years I've been completely focused on two goals: playing my best and helping the Yankees win. That means that for 365 days a year my every thought and action were geared toward that goal. It's now time for something new."

The 39-year-old continues, "I want to soak in every moment of every day this year, so I can remember it for the rest of my life. And most importantly, I want to help the Yankees reach our goal of winning another championship."

Back in November, Jeter agreed to take a pay cut ... and signed a 1-year deal for 2014 that will pay him $12 million.