The discovery of the “extraordinary” dolmen occurred just as the archaeologists were about to close up an excavation site at the Don Bosco celtic burial grounds in the city of Sion.
The researchers had been monitoring and guiding construction during foundation works for a new garage in the north of the city when they came upon a number of massive stone slabs weighing several tonnes.
The slabs are part of a 5,000-year-old dolmen that could contain the remains of hundreds of bodies, cantonal authorities said in a statement.
Investigations will now be carried out to try and date the dolmen more precisely.
Researchers will also examine the stone slabs to see if they were engraved and try and ascertain if human remains are still present, or if these have been washed away during flooding of the La Sionne river.
The finding of the dolmen is reminiscent of the discovery of another megalithic tomb unearthed in the Petit-Chasseur district of Sion at the beginning of the 1960s.
That dolmen and its engraved stele are considered prehistoric masterworks of Europe-wide significance, Valais authorities said in their statement.
Archaeologists will now try and place the new find in the context of other discoveries in the city.