Kim Kyung-il was the captain of the first coast guard ship sent to help passengers on the Sewol vessel as it sank last April.
He was found guilty of professional negligence resulting in death, a court spokesman said.
More than 300 passengers died, most of them school students.
Prosecutors said Kim bore responsibility for the botched rescue effort that wasted precious time and delayed the evacuation of people from the vessel.
He was also charged with making a false report that he had broadcast an evacuation order through loudspeakers.
The court said fewer people would have died if he had ordered them off the ship straight away.
The cause of the disaster has been attributed to overloading by the ship's owners.
However, the rescue response was also criticised for being slow, unco-ordinated and unfocused, and prompted President Park Geun-Hye to vow a complete overhaul of national safety standards.
More than 50 people have been put on trial on charges linked to the sinking, including 15 crew members who were among the first to climb into lifeboats.
The Sewol's captain was jailed in November for 36 years for gross negligence and dereliction of duty, while three other senior crew members were sentenced to jail terms of between 15 and 30 years.
Billionaire businessman Yoo Byung-eun, who owned ferry operator Chonghaejin Marine, went missing shortly after the disaster, sparking a massive manhunt.
He was found dead in June by police.
A former coast guard captain has been jailed for four years for negligence in connection with the rescue operation for a stricken South Korean ferry.