Lonzo Ball is suing Alan Foster -- the co-founder of the Big Baller Brand -- alleging the longtime family friend conned them and swindled them out of more than $1.5 million.
TMZ Sports has obtained the lawsuit filed in Los Angeles on Tuesday ... in which Ball alleges Foster orchestrated a "fraudulent scheme" to take money from BBB and divert the money to his personal accounts.
Ball claims Foster intentionally got close to his family knowing he was one of the best basketball players in the country and would likely go on to make millions in the NBA.
The Balls also claim they had no knowledge of Foster's checkered past ... and that he spent several years in federal prison in the early 2000s for his alleged role in a ponzi scheme.
Ball claims Foster began conspiring against them shortly after his release.
It was reported last week that Foster -- who owns around 16 percent of the company -- allegedly stole the money and that Ball cut ties with him ... so it was only a matter of time before it went to court.
Zo also alleges when Foster was confronted, he threatened to expose the Ball family with false information to the public.
It's unclear how much Lonzo is asking for ... but it's well in excess of $2 million.
We just spoke with LaVar this weekend ... and he addressed all the speculation that they were shutting down the family business and ending the BBB for good -- but he said that's simply not the case.