James penned the words in a lengthy social media post Monday night ... addressing Bryant's death for the first time since Mamba, his daughter, Gianna and 7 other people were killed in a tragic helicopter crash Sunday.

"Man I sitting here trying to write something for this post but every time I try I begin crying again just thinking about you, niece Gigi and the friendship/bond/brotherhood we had!" James said.

"I literally just heard your voice Sunday morning before I left Philly to head back to LA. Didn’t think for one bit in a million years that would be the last conversation we’d have."

James and Bryant had a friendship that dated back more than a decade ... the two were competitors in the NBA for years and were teammates on Team USA at the 2008 and 2012 Olympics.

The two obviously grew closer when LeBron left Cleveland for Los Angeles in 2018 ... and, in fact, Kobe's last-ever post on social media was congratulating Bron for passing him on the NBA's all-time scoring list over the weekend.

"Man I love you big bro." Lebron said in his letter to Kobe. "My heart goes to Vanessa and the kids. I promise you I’ll continue your legacy man! You mean so much to us all here especially #LakerNation💜💛 and it’s my responsibility to put this sh*t on my back and keep it going!!"