(1729–79, or Kyōhō 13–An'ei 13) was an Edo period Japanese pharmacologist, student of Western studies, physician, author, painter and inventor who is well known for his Erekiteru (electrostatic generator), Kandankei (thermometer) and Kakanpu (asbestos cloth). He also wrote the satirical essay "On Farting."

Born into a low-ranking samurai family, his father was Shiraishi Mozaemon (Yoshifusa), his mother was from the Yamashita clan, and he had many siblings. His real name was , but he also went by the pen names , (his principal literary pen name), and . He is most well known by the name "Gennai," however.

He first studied medicinal herbs in Osaka, with Toda Kyokuzan, before moving to Edo in 1757. There, he studied with Tamura Ransui, and wrote a number of books, some on scientific or nature topics, some satirical novels, in the kokkeibon and dangibon genres. In his scientific experiments, he prospected for various ores, weaved asbestos, calculated temperatures, and worked with static electricity. Gennai also studied Western painting and ceramics techniques, and produced a number of works in that vein.

Interested in ores, he tried unsuccessfully a number of times to have new mines opened. On one occasion, frustrated and enraged at the repeated lack of support from the citizens of the area, he killed one of his disciples in a fit of rage. Arrested and sent to prison, he died there in 1779.

Appearances in fiction

• In the anime OVA: 'Mask of Zeguy' Hiraga Gennai had a prominent role (along with Hijikata Toshizo) in preventing the legendary mask and protecting Miki(who is a descendant of the renowned Priestess Shamus)from falling into the wrong hands

• In the anime Oh! Edo Rocket episode 10 it is revealed that the retired resident is Gennai. The Fūrai Row-House Block, which he says is his, is also likely a nod to one of his pen names.

• In the anime Gintama, there is a mechanic known as Hiraga Gengai.

• The anime Zero no Tsukaima has a character by the name of Hiraga Saito. Since Saito hails from Japan, it is speculated that he is named after Gennai.

• Gennai makes an appearance in the anime Read or Die, along with many other historical and legendary figures. In Read Or Die, Gennai uses his elekiter as a very high powered destructive weapon that he uses to destroy the White House and eradicate an entire fleet of combat helicopters.

• He is also referenced in Mai-HiME.

• In the anime Flint the Time Detective, he makes an appearance with the Time Shifter Elecky as he uses it to make giant robots.

• In the Squaresoft game Live-A-Live, there is a mechanic named Gennai who is responsible for the creation of mechanical traps in the Bakumatsu Chapter. Since the setting of the chapter is the Bakumatsu era, his presence is an anachronism, but considering the additional presence of Ishikawa Goemon, Yodo-Dono, and Shiro Tokisada Amakusa, it is clear that this section of the game was intentionally designed as a mash-up of popular Japanese history.

• In the 36th episode of "Kikaida 01" Hiraga Gennai is threatened by time traveling robots from 1974 disguised as ninja. The evil Shadow tends to take him to 1974 and have him help build better robots.

• In the 30th episode of "Demashita! Powerpuff Girls Z" (ガールズとカレ! - Girls and Him!), a character by the name of Hiraga Kennai is responsible for the creation of a primitive form of Chemical Z and the Ōedo Chakichaki Musume. He also uses an elekiter to separate Him's soul (the black light) from his body.

• In the 13th episode of the first season of the anime Digimon Adventure (エンジェモン覚醒! - Angemon's Awakening!), an elderly man named Gennai appears to the Chosen Children/Digidestined and helps them with their journey. He reappears in the second season called Digimon Adventure 02 as a younger man. His Digimon Adventure design appears to be based on old-fashioned Japanese styles, and both it and his name were likely inspired by the historical Gennai.