A historic mansion on Manhattan’s Fifth Avenue Gold Coast is in contract for $75 million — which would be the highest price ever paid for a co-op in the city, The Post has learned.

The buyer of 828 Fifth Ave. is Russian billionaire Roman Abramovich and his stunning baby mama, Dasha Zhukova, a source said.

The couple had been secretly eyeing the city’s top-priced town houses for the past year.

They have two US-born children. Abramovich has five more with his ex-wife.

The current record co-op price is the $54 million David Geffen paid for socialite Denise Rich’s pad at 785 Fifth Ave.