- Alternate Spellings: Portunes, Portumnes, and Portunes
- Iconography: A man (possibly two-headed) with a key in his hand.
- Presides over: Keys, gates, doors, harbors, fords, warehouses, and livestock
- Holy day: August 17 (some sources cite August 16), the Portunalia, when one throws a key into a fire to ensure good luck.
Portunus protected the warehouses where grain was stored. Probably because of folk associations between porta "gate, door" and portus "harbor", the "gateway" to the sea, Portunus, who also believed to have influence over the timing of waves and thus could dictate the moments and events in the lives of those on or near the sea. He later became associated with Palaemon and evolved into a god primarily of ports and harbors.
In the Latin adjective importunus his name was applied to untimely waves and weather and contrary winds, and the Latin echoes in English opportune and its old-fashioned antonym importune, meaning "well-timed' and "badly-timed". Hence Portunus is behind both an opportunity and importunate or badly-timed solicitations.
Linguist Giuliano Bonfante has speculated, on the grounds of his cult and of the meaning of his name, that he should be a very archaic deity and might date back to an era when Latins lived in dwellings built on pilings. He argues that in Latin the words porta (door, gate) and portus (harbour, port) share their etymology from the same IE root meaning ford, wading point.
His festival, celebrated on the seventeenth day before the Kalends of September, was the Portumnalia, a minor occasion in the Roman year. On this day, keys were thrown into a fire for good luck in a very solemn and lugubrious manner. His attribute was a key and his main temple in the city of Rome, the Temple of Portunus, was to be found in the Forum Boarium.
Portunus appears to be closely related to the god Janus, with whom he shares many characters, functions and the symbol of the key. He too was represented as a two headed being, with each head facing opposite directions, on coins and as figurehead of ships. He was considered to be "deus portuum et portarumque praeses."