The deal will add to the 66,000 square feet the company already leases at the building, the 600,000-square-foot property’s entire fourth floor.

AppNexus will take the new space for five years in the deal for rents in the high $50s per square foot.

AppNexus's Fifth Floor Offices At 28-40 West 23rd Street

“We are delighted to be able to accommodate AppNexus’ growth and expansion in the building,” Andrew Roos, an executive with Colliers International who handled the lease, said in a statement.

Mr. Roos is a part owner of the property, along with the Cohen and Carmel families.

“This complex has always had a diverse tenant roster, and with the explosion of the tech sector, we find ourselves at the hub of the digital and media industries, which are increasingly drawn to Midtown South,” Mr. Roos said, referring to the tenants groups that have sparked Midtown South’s dramatic success in recent years even as other neighborhoods like Midtown have seen stagnant rental and occupancy growth.

AppNexus is one among a few tech companies and creative firms that have come to 28-40 West 23rd Street. Earlier this year, a digital division of Estee Lauder took the property’s entire third floor, another 66,000 square foot space. Big Fuel, an online marketing company that was recently acquired by Publicis, subleases space from Ecko, the building’s former anchor tenant. Shoe and apparel maker Converse also occupies space.

The building has appeared to follow in the footsteps of other pioneering success stories in the area, including 200 Fifth Avenue and 111 Eighth Avenue.

“I looked at the success of 111 Eighth Avenue and 200 Fifth Avenue and basically wanted to emulated that,” Mr. Roos told The Commercial Observer during a recent tour of the property. “Those are the bookends of the neighborhood and I’m kind of in the middle.”