Jack J. Grynberg (January 21, 1932 – October 11, 2021) was an American businessman. He was a Denver-based oil and gas developer who amassed a multibillion-dollar fortune in the oil and natural gas sector. As of 2019, he was in a long-running litigation to regain control of companies he founded from his ex-wife and three children, or failing that, around $400 million in back pay.