The island proposal could be fully established by 2070, host housing for 35,000 people and also protect the city from rising sea levels, Lars Lokke Rasmussen said at a press briefing.
"The capital is in rapid development, being built as never before. That's good but it also means that Copenhagen will run out of plots eventually, and that's why we want to build a new island," Rasmussen said.
The liberal prime minister was joined at the briefing by Copenhagen's Lord Mayor Frank Jensen, a social democrat, indicating support for the project from the main opposition party in the national parliament.
The project, tentatively called Lynetteholm, would demand investment of around 20 billion Danish crowns ($3.1 billion) including in a harbor tunnel and a metro line, but will be self-financing over time, the prime minister said.
The project would be a subsidiary of the CPH City & Port Development, a company jointly owned by the City of Copenhagen and the Danish State.