By Agence France-Presse
The chopper was hovering over the Han River in eastern Seoul to draw water into its tank when it went down, they said.
The craft had been deployed to fight a bushfire on nearby hills.
Two pilots survived with injuries but a mechanic was killed.
The wildfire on Mount Yeongchuk has since been under control.
Police said they were investigating.
"Two people are still recovering from injuries, so it's difficult to get their statements at this moment," police said according to Yonhap news agency.