Twenty people, including 19 oil rig workers and the co-pilot, were rescued but the 48-year-old pilot was still missing.
Terengganu state police chief Acryl Sani Abdullah Sani said the helicopter is thought to have experienced technical problems and crash-landed about 200 metres (656 feet) from a platform belonging to US oil giant ExxonMobil.
"The colleagues of the workers on the platform realised what had happened and immediately launched a rescue operation," he was quoted as saying by the national Bernama news agency.
The crash occurred in the early afternoon some 113 nautical miles from the coastal town of Dungun, a local police officer told AFP.
"Thirteen passengers were injured," the officer said.
Three of the workers are believed to have been flown to a hospital in the state's capital of Kuala Terengganu, said Bernama.
Acryl said Malaysia's Maritime Enforcement Agency was searching for the pilot.

Copyright AFP (Agence France-Presse), 2006