SOCAR said a small cabin, where workers were living, was washed into the sea from the platform during a heavy storm.
"The accident did not affect the oil production process," the SOCAR spokesman, Nizameddin Guliyev, told reporters.
He said that a search-and-rescue operation was underway.
Four workers were killed and one went missing after a fire broke out on SOCAR's other oil and gas platform in the Caspian Sea in Oct. 2014.
(Reporting by Nailia Bagirova; writing by Margarita Antidze; editing by Jane Merriman)