Dentsu Americas, the US arm of the Tokyo-based ad firm, is making a play for the largest floor at the Farley Building at 390 Ninth Ave., sources say.

The US Postal Service building is being redeveloped by Vornado Realty Trust into 740,000 square feet of office space on its four top levels with lobbies, tons of retail and the new Moynihan Train Station — Penn Station — below.

The historic building sits on the entire superblock bounded by West 31st and 33rd streets and Eighth and Ninth avenues.

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The fourth floor Dentsu is eyeing spans 292,692 square feet, with the remaining three ranging from 100,000 to 180,000 square feet.

Dentsu would move here from Rudin’s 1.1 million square feet at 32 Sixth Ave., sources said, when its lease expires around 2022. It has been expanding there but is landlocked as this large landmark with unique global connectivity is nearly full.

Dentsu is represented by Cushman & Wakefield and the deal is still preliminary. But the possibility of a pending deal at Farley is creating a sense of urgency for other large tenants, like Amazon, seeking to lease creative, tech-friendly blocks of space.

Amazon has long been wooed by Vornado for the top of the Farley Building. The horizontal space can be divided to include private lobby entrances for three separate tenants.

Apple and Facebook have already toured the space at the Farley Building, sources said.

Amazon is raring to get to work in the West 34th Street neighborhood, where it already has other offices, including one at Five Manhattan West, owned by Brookfield.

As The Post reported Monday night, Amazon is also taking a hard look at 2 Manhattan West through JLL. That need is for as much as a hefty 750,000 square feet, which would be good news for whichever building lands it.

The Farley is slated to be completed sometime in 2020.