On the ancient Roman calendar, mensis Iunius or Iunius, also Junius (June), was the fourth month, following Maius (May). In the oldest calendar attributed by the Romans to Romulus, Iunius was the fourth month in a ten-month year that began with March (Martius, "Mars' month"). The month following June was thus called Quinctilis or Quintilis, the "fifth" month. Iunius had 29 days until a day was added during the Julian reform of the calendar in the mid-40s BC. The month that followed Iunius was renamed Iulius (July) in honour of Julius Caesar.

The Roman calendar was the calendar used by the Roman Kingdom and Roman Republic. Although the term is primarily used for Rome's pre-Julian calendars, it is often used inclusively of the Julian calendar established by the reforms of the Dictator Julius Caesar and Emperor Augustus in the late 1st century BC.