Professor (Reg) Chronotis is a fictional character created by Douglas Adams. He originally appeared in the Doctor Who story Shada, starring Tom Baker and Lalla Ward. However, since the filming of the latter was never completed due to industrial action, the public were not introduced to him until the publication of Adams' novel Dirk Gently's Holistic Detective Agency in 1987. (An incomplete version of Shada was eventually released on VHS, and in 2003 it was remade as an animated webcast by the BBC.)

Reg Chronotis holds the post of Regius Professor of Chronology at Cambridge University. He is also a Fellow of the fictional St. Cedd's College, Cambridge, where he has resided for many hundreds of years. In the Shada version, Chronotis is a retired Time Lord and old friend of Doctor Who, living out his remaining centuries in academic seclusion. He is extremely forgetful and absent-minded and has a liking for tea and for silly jokes.

The Dirk Gently version of the character is almost identical to the Shada version, though the novel contains no references to Time Lords. Here Chronotis is so old and forgetful that he has no idea who or what he originally was, though he has vague memories of Cleopatra (who he claims wore outrageous earrings and reeked of cat food). His Chair of Chronology was created by mad king George III who was terrified that if time were to start flowing backwards, all the bad experiences of his life might reoccur.

In both versions, Chronotis is a clandestine time-traveller, whose time machine (or TARDIS in the lingo of Doctor Who) is disguised as his College rooms.