Almost 50 skeletons dating to the 7th and 11th centuries have already been uncovered during two previous digs at St Patrick's Chapel in Pembrokeshire.
Dyfed Archaeological Trust will carry out a final excavation of the site in the dunes at Whitesands Bay, St Davids, from 9 to 27 May.
It will also be offering free tours.
Many of the skeletons found were in 'cist' graves - long graves lined with stone slabs.
Child graves were also found, decorated with layers of quartz pebbles and limpet shells.
Ken Murphy, of Dyfed Archaeological Trust, said bone preservation in the first two digs had been very good.
He also said a stone cross was found standing at the end of one of the graves - the first time in Britain that an upright stone cross had been found in association with a long cist grave.
"Bone analysis by the University of Sheffield will provide information on the diet of the people who are buried at St Patrick's Chapel, and even on where they were born," he added.
"The results of the excavation will greatly advance our knowledge of the lives and the beliefs of the people who lived in Wales over 1,000 years ago."
The chapel, from where St Patrick is said to have set sail for Ireland in the 5th century AD, was a ruin over 400 years ago.
But its location has never been forgotten and graves with human remains have regularly been exposed by storms.
The Pembrokeshire Coast National Park Authority placed boulders against the site of the chapel to halt erosion 12 years ago, but they were washed away in the winter storms of 2014, exposing more burials.
"So we then decided to excavate the parts of the site most vulnerable to erosion and the aim of these digs is to ensure no more archaeology will be at risk for the next 50 years," added Phil Bennett, the park authority's culture and heritage manager
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Archaeologists hope to find out more about people living in Wales 1,000 years ago as they excavate an early medieval chapel on a beach.