The ancient Roman god who gave January its name is Janus.
Janus was associated with beginnings, transitions, doorways, gates, and passages. Roman artists often represented him with two faces, one looking forward and one looking backward. That imagery later became especially fitting for New Year's celebrations, which look back on the completed year while anticipating the next.
The Roman month of January, Latin Ianuarius, was named for Janus. The month became the first in the Roman calendar after calendar reforms, although Roman calendars changed over time and January was not always treated identically in earlier systems.
Janus is distinctive in Roman religion because he does not have a direct Greek equivalent. His role is linked to thresholds and change rather than to a single natural force. The modern word “January” preserves his name, making the connection between the god and New Year's observances visible in everyday language.