Marty Ingels -- the husband of the legendary Partridge Family matriarch Shirley Jones -- is under investigation for allegedly shoplifting a $500 hat.

It happened at Baron Hats, a custom store that has made hats for the "Indiana Jones" and "Pirates of the Caribbean" movies. We're told Ingels had an ongoing dispute with the store over a certain hat.

Ingels allegedly went into the store on Monday, got into a tiff with a clerk, picked up the hat in dispute and then walked out of the store.

Ingels was in such a rush he left his cane behind.

Marty tells us the clerk is the one who put the store hat in his pile. He says he returned it late Tuesday afternoon.