LaVar Ball -- the father of basketball stars Lonzo, LiAngelo and LaMelo Ball -- had his foot amputated after suffering a serious medical issue, TMZ Sports has learned.

We're told the Ball family patriarch underwent a recent procedure to have his right foot removed ... but despite it all, he is in great spirits and doing well.

LaVar made a name for himself with his grandiose plans for his sons -- he created the Big Baller Brand, which developed signature shoes for all three hoopers.

LaVar made countless headlines for his bold declarations about his kids, but has received praise following their success -- Zo and Melo are stars in the Association, and Gelo's music career is taking off.

He has distanced himself from the spotlight in recent years ... but has continued to be his sons' biggest fan from afar.

LaVar was a college sports star in his own right -- he played at Cal State-Los Angeles ... and spent time with the New York Jets and Carolina Panthers organizations.