The accident occurred in Bang phii area of Samut Songhkram province, killing 11 people, authorities said.
The beam collapsed and fell on workers who were having their lunch at the building, which was a part of the expansion of a public hospital.
Some of the injured were in critical condition, hospital sources said.
Rescuers were checking for survivors who may still be trapped under the rubble at the construction site.
Hospital staff said the beam linked two buildings. The rescue work was dangerous as there was a risk that the entire structure could collapse.