St Albans District Council has said the 30 coins were taken from a locked display cabinet in the Medieval Gallery at the Hatfield Road museum.
It is thought the locks on the cabinet were broken before the coins, estimated to have an insurance value in the region of £12,000, were taken.
The council says the theft occured on or around the weekend of Saturday, January 7.
A sixth or seventh century silver hand pin, discovered in excavations at St Albans Abbey, were also taken from the same case.
The coins were part of a hoard of Saxon silver coins found at the Abbey Orchard in St Albans in 1969 during an excavation by the St Albans and Hertfordshire Architectural and Archaeological Society, within the area of the monastic buildings attached to the Abbey, prior to the construction of Abbey Primary School.