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.