| Named By: | Robert R. Reisz in 1972 |
| Time Period: | Late Carboniferous, 306 Ma |
| Location: | Canada, Nova Scotia. Czech Republic, Nyrany. USA, Ohio |
| Size: | 50 centimetres long |
| Diet: | unavailable |
| Fossil(s): | Three specimens |
| Classification: | | Chordata | Tetrapoda | Synapsida | Pelycosauria | Ophiacodontidae | |
Archaeothyris is an extinct genus of ophiacodontid synapsid that lived during the Late Carboniferous and is known from Nova Scotia. Dated to 306 million years ago, Archaeothyris, along with a more poorly known synapsid called Echinerpeton, are the oldest undisputed synapsids known. Protoclepsydrops also from Nova Scotia is slightly older but is known by very fragmentary materials.