The two-time All-Star was previously suspended for eight games in March after he appeared in another social media video holding a firearm at a nightclub.
The Grizzlies suspended Morant from team activities in May over the latest incident, in which he appeared to pose with a firearm in a car, conduct that the NBA found to be "detrimental to the league."
"Ja Morant's decision to once again wield a firearm on social media is alarming and disconcerting given his similar conduct in March for which he was already suspended eight games," NBA Commissioner Adam Silver said in a statement.
"The potential for other young people to emulate Ja's conduct is particularly concerning.
"Under these circumstances, we believe a suspension of 25 games is appropriate and makes clear that engaging in reckless and irresponsible behaviour with guns will not be tolerated.
"For Ja, basketball needs to take a back seat at this time. Prior to his return to play, he will be required to formulate and fulfil a program with the league that directly addresses the circumstances that led him to repeat this destructive behaviour."
Morant took accountability for his actions last month and released another statement after the suspension was announced, apologising to the NBA, the Grizzlies and the city of Memphis.
"I'm sorry for the harm I've done. To the kids who look up to me, I'm sorry for failing you as a role model. I promise I'm going to be better," Morant said in a statement reported by ESPN.
"To all of my sponsors, I'm going to be a better representation of our brands. And to all of my fans, I'm going to make it up to you, I promise.
"I'm spending the off-season and my suspension continuing to work on my own mental health and decision-making. I'm also going to be training so that I'm ready to go when I can be back... I'm so sorry I won't be out there with you (teammates) at the beginning of the season."
The suspension was effective immediately and remains in effect through the first 25 games of the 2023-24 NBA regular season.
