The 50-year-old man was working on the McCarthy & Stone building site at the seaside town’s old fire station when his leg was crushed between two pieces of steel.
It is believed the incident involved heavy sheet piling.
A spokesman for the retirement home developer said: “McCarthy & Stone confirms that an accident involving a piling subcontractor occurred at its construction site in North Quay, Weymouth, at around 11.00am on Wednesday morning.
“Site employees followed procedures and emergency services arrived quickly. The subcontractor was then taken to Southampton General hospital via air ambulance.
“The Health and Safety Executive has been informed and the company is working with the relevant authorities, as well as carrying out its own investigation into what happened.”