Beyonce's Dad -- You're a Deadbeat ... Baby Mama Alleges

Beyonce's father is a DEADBEAT DAD ... at least according to the mother of his 3-year-old son who claims Mathew Knowles has stiffed her out of $24k in child support, TMZ has learned.

According to California State records obtained by TMZ, Mathew missed TWO of the $12k monthly payments he's been ordered to pay to Alexsandra Wright.

Sources close to Wright says she's in a financial bind and needs the cash asap -- we're told she's even had to pull her son from school because she can't afford to pay for it.

As we've reported ... the legal battles between Knowles and Wright have been ugly. She claims Knowles sent a threatening email to her in 2011 ... warning "there's a price to pay" for hurting his family.

We reached out to Mathew for comment -- so far, no word back.