Post Malone Rocks Karl Malone Jersey During Concert W/ NBA HOF'er In Stands

Karl Malone is a big fan of Post Malone -- and the feeling's apparently mutual, 'cause the rap star rocked the Hall of Famer's Jazz jersey during a recent concert in Utah ... and the Mailman, who happened to be at the show, absolutely loved it!

The Jazz posted video of Karl, 59, at Post's concert at Vivint Arena in Utah ... showing the "Congratulations" rapper getting showered with cheers from the raucous audience. The camera then pans to KM ... who has clearly jazzed the rapper rocked his jersey.

Of course, Karl rocked the iconic #32 jersey throughout his legendary NBA career.

And get this ... after the concert, Karl and Post linked up backstage, where they played beer pong! No word who won.

The Jazz selected Karl in the first round of the 1985 NBA draft ... and he spent 18 seasons with the franchise, beating up on the league for nearly two decades with running mate, John Stockton.

Although Karl never won an NBA title (blame Michael Jordan), "The Mailman" is recognized as one of the best power forwards in league history.

Malone was inducted into the Naismith Memorial Basketball Hall of Fame in 2010 twice --- for his career with the Jazz and as a member of the 1992 U.S. men's Olympic basketball team.