Jacob Franklin Mentzer, III (born 1950 in Philadelphia) is an American game designer, game collector, writer, and former folk musician who is most famous for designing the Dungeons & Dragons boxed rules sets that appeared in the 1980s. He worked at TSR from January 1980 until October 1986 and became a good friend of Gary Gygax. In 1981, he founded the Role-Playing Games Association (RPGA). Since 1983, he has been involved with the world's largest game auction at the GenCon game convention.
Mentzer performed as a folk musician between 1968 to 1975 and even played one concert at the White House during the Nixon administration. He has recently been living in Minocqua, Wisconsin, where he has been working as the manager of a bakery. His sister is mezzo-soprano opera singer Susanne Mentzer.
He worked as Gary Gygax's right-hand creative man for so many years which helped him learn how to write in Gary Gygax's unique writing style sometimes known as "High Gygaxian" or "Higax".