Tambon Na-in Administrative Organisation president Ramian Chandam reported the staircase discovery at Khao Khitone in Ban Fak Beung.
He said his office staff, along with some villagers, had cleared the area, in preparation to developing the site into a new tourist destination.
The staircase leads to the ancient compound with a beautiful view of Pichai district.
Ramian said his office had obtained funds to build a road linking villages and project workers had discovered the stairs while using a heavy machine to dig up bamboo covering up the staircase entrance.
Archaeologist Rattiya Chaiwong, who led the Fine Arts Department's Archaeology Office 6 in Sukhothai team to inspect the hilltop, said the Buddhist site and its laterite staircase were of the same era.
They were possibly the home of monks connected to communities in Pichai district area.
The discovery of ceramics produced by Sing Buri's Mae Nam Noi kilns and Sangkhalok glazed pottery of Sukhothai's Si Satchanalai kilns also confirmed the site's origin as of the 20th-21st century of the Buddhist era (BE 1901-2100), she said.
A broken sandstone inscription from the Sukhothai era, found at the site, had been sent to the National Library in Bangkok, she added.