Rapper Juelz Santana is a deadbeat dad ... so say authorities in NJ looking to nail him for a ton of back child support.

Juelz owes nearly $30k to his baby mama for their 13-year-old son, LaRon Jr. ... according to legal docs obtained by TMZ. Juelz was ordered to pay $1,800 a month back in 2013, but you could say he's fallen a little behind -- since Jers says he owes $28,491.32.

His next payment is due on the 16th and there's an active warrant out for his arrest.