Janus was, of all the Roman gods, the most distinctively Roman — a god with no Greek counterpart, whose office and iconography were unique. He was the god of doorways (ianua, his sacred word, gives us our "janitor" and "January"), of beginnings, of transitions, of the moment between past and future. He was depicted with two faces, one looking forward and one looking back, that he might see what was to come and what had been. His month was January, the first month of the Roman calendar after Caesar's reform, named for him. The first day of every month was sacred to him; the first hour of every day belonged to him; the first invocation of any prayer was made to him before any other god, and even Jupiter received second mention. He had no temple in the conventional sense; instead, he had the Janus Geminus, a small ceremonial gateway in the Forum (sometimes called the temple of Janus, though it was really an arched passage with two doors), which stood open in time of war and was closed only when the Roman state was at peace. The closures were rare events of historical note: the door was closed only twice between the founding of the city and the time of Augustus, who closed it three times during his reign as a sign of the Pax Augusta.

His mythology was sparse but distinctive. He had been, in the antiquarian tradition, a king of Latium in the time before Rome's founding, who had received the exiled Saturn (driven from heaven by Jupiter) and learned from him the arts of cultivation; he had founded the citadel on the Janiculum hill (which bears his name) and reigned wisely. He had a wife, Camasene (or Camise), and a son Tiberinus who gave his name to the Tiber. He had no extended narratives in the Greek manner; his significance was ritual and conceptual rather than dramatic. He was the god of opening — and the Romans, who had a deep instinct for the importance of beginnings, gave him pride of place in their religion accordingly.