Tyler Robinson's dad doesn't seem to be hurting for cash ... because he runs a successful local masonry business.

According to the California Post, Matthew Robinson is back to work in St. George, Utah after his son spent days in court for the alleged murder of Charlie Kirk.

The outlet reports that folks who have done business with Tyler's pops say business is booming ... thanks to loyal clients and word-of-mouth.

Tyler's dad isn't cutting the checks for his son's defense ... taxpayers are on the hook ... and experts recently told the CA Post earlier this week that the bill could get up to $10 million, maybe more.

According to the outlet, things haven't always been so good for Matthew's business, citing a 2014 lawsuit where his brother, Clint Robinson, accused him of embezzling $100K. The siblings ultimately came to a confidential settlement.

One of Matthew's colleagues -- who asked to remain anonymous -- told the Post the guy he doesn't mention Tyler much these days.

However, we have seen Matthew come to his son's recent court hearings to show support.