Mercedonius was a month in the ancient Roman calendar. Also known as Intercalaris it was a intercalary month of 22 days, being inserted into the month of Februarius only in leap years. Romans believed that the month had been added to Roman calendar (along with Januarius and Februarius) by the king Numa Pompilius in the 7th century BC.

Originally this month was inserted every two years to realign the Numa's 355 day calendar with the seasons. Later a system inserting several months every eight years was adopted from the Greek calendar. Neither system reflected the true length of the year very well and priests often forgot to insert the months at the proper times. Additionally, for political reasons the lengths of the intercalary months were often deliberately varied by priests to allow certain consuls to stay in office longer and force others out earlier.

The month was eliminated by Julius Caesar when he introduced the Julian calendar in 45 BC. The name "Mercedonius" comes from merces meaning "wages" as workers were paid at that time of year.