Cantonal police said the incident happened at around 10.30am when for reasons as yet unexplained a rolling platform with scaffolding toppled six metres into the lake.
The man who was fatally injured died on the spot despite attempts by colleagues and emergency services workers to revive him, police said.
Three other workers on the scaffolding also ended up in the lake and were injured.
They were transported to hospital for treatment, police said.
Officials launched an investigation into the accident, which occurred in the municipality of Euthal, about 40 kilometres southwest of Zurich.
The bridge project, known as the Steinbach Viaduct, is a 30-million-franc project to replace an existing crossing of the lake between Euthal and Steinbach.
Construction on the new 441-metre bridge, capable of handling heavier traffic loads than the existing span, was scheduled to be finished in the late summer of 2014.
It was not immediately clear if this accident will affect the schedule.