Dwyane Wade's father wasn't always banking on his son becoming a gazillionaire ... because he took out a loan to attend trade school back in the day ... a loan he allegedly never re-paid.
TMZ has learned ... D-Wade Sr. is being sued by the U.S. Govt. for defaulting on a $4,630.46 loan intended for higher education.
According to the docs, Wade Sr. borrowed the cash to attend the Environmental Technical Institute -- which specializes in the heating, air conditioning and refrigeration industry.
The loan was approved in 2001 -- the same year Dwyane Wade Jr. entered his freshman year at Marquette University.
The U.S. Govt. is going after Wade Sr. for the full amount of the loan -- plus interest -- totaling $6,221.44 ... which DW Sr. could probably find in his son's couch cushions.
Attempts to reach Wade Sr. for comment were unsuccessful.