Officials rushed to evacuate people and their belongings to higher ground as work crews piled up more than 5,000 sandbags along the riverbank to stem the flood.
Reports said the floodwater had spread into the provincial municipality and authorities had to turn off power in some areas for safety.
Most roads in the provincial capital inundated and in some areas the water was still continued rising and was over one metre high. Traffic was paralysed.
Udom Darunee School, Wat Thai Chumpol School, Wat Khuha Sawan School, Kwang Tong School, and Sai Tham School had been temporarily closed.
Wichian Chamong, Ratchathani, Pracha Ruam Jai, Khlong Taphet, Phra Mae Ya, and Tan Dieo communities in the the municipality were heavily affected.
The grounds of city hall, the Provincial Administration Organisation office, prosecution office, Muang district office, and municipality office were under about 50 centimetres of water.
Sukhothai governor Chakarin Plienwong sent rescue teams to help flood-hit villagers, and ordered huge sandbags to plug holes in the embankment.
The reports said water from Phrae province is running into the Yom River at a rate of 700 cubic metres per second.
People in the four districts of Sukhothai -- Sawankalok, Muang, Sri Samrong and Kong Krailas -- have been warned of more flooding to come.