Bob Marx is the alias used by American voice actor R. Martin Klein for much of his dubbing work in anime during the 1990s. Klein, who is also credited as Robert Martin Klein and Bob Klein, has had a prolific career in voice acting spanning from 1981 to the present.

His body of work is extensive, particularly in the anime genre, where he is known for a wide range of characters. Among his most recognized roles are Ivan Whisky, also known as Cyborg 001, in the series Cyborg 009 The Cyborg Soldier. This role is frequently cited as one of his notable performances under the Bob Marx pseudonym. Another key role from this period is Kurageman in the series Hyper Doll.

Beyond these two examples, Klein's career under both his own name and the Bob Marx alias includes a vast number of other anime credits. He is particularly well-known for his long-standing association with the Digimon franchise, voicing Gomamon and its evolutions, including Ikkakumon, across multiple series and films such as Digimon: Digital Monsters, Digimon Adventure tri., and Digimon Adventure: Last Evolution Kizuna. His other notable anime voice roles include Katsuhiko Jinnai in El Hazard, Fukurou in Noein, Gaku in FLCL, Flint Hammerhead in Flint the Time Detective, and Mr. Me-Mania in the film Perfect Blue.

In addition to anime, Klein has contributed his voice to various non-anime projects, including animated series like Animalia, video games such as the.hack series and Star Ocean: First Departure, and even a voice cameo in the live-action film Meet the Fockers. His work demonstrates a recurring collaboration with voice direction and production teams active in English dubs during the 1990s and 2000s. Klein's long-standing and varied career, marked by iconic roles in several major franchises, stands as a notable achievement in the field of voice acting.