It's not official yet since he has not announce it yet, you can read this article entitled Is Dirk Nowitzki going to retire? The NBA is acting like he is and gain more information. He is on his farewell tour like Dwayne Wade but I think this would be the last season of wade as he is set for retirement.

Dirk had already contributed a lot of entertainment in the NBA, he gave one championship to the Mavericks so he is a fulfilled player which has nothing to prove anymore.