And it wasn’t just him they were there to see.

Scenes at the show’s fictional “Arconia” building—where the three main characters live and sleuth—are shot at the real-life Belnord, a condominium on the Upper West Side of Manhattan. Wrapped around a roughly 22,000-square-foot courtyard, the Italian Renaissance building takes up on an entire block. Since “Only Murders in the Building” made its debut in August 2021, fans have started visiting the building, posting photos of the now-iconic facade and courtyard on Instagram and Twitter with hashtags referencing the show.

“The Belnord has become a bit of a celebrity, with people dropping by and taking photos,” said Mr. Martin.

Belnord resident Lisa Lippman, a broker at Brown Harris Stevens, said the building’s newfound fame “has caused a stir with all the younger people walking in, all because of the show.”

And while the Belnord’s starring role on the popular mystery series hasn’t translated to an appreciable uptick in real-estate sales at the building, agents said, that hasn’t stopped them from playing up its connection to “Only Murders.”

“Having the show as a brochure is definitely a plus,” said buyer’s agent Nicole Dreyfuss of Triplemint, who recommended that her client, Anne Douglas, watch the show after touring a unit in the building. The following weekend, Ms. Douglas texted her: “I’ve been watching ‘Only Murders in the Building’ and it has me falling in love…I can’t stop thinking about the courtyard views.” Ms. Douglas, 57, said she is now in contract on a three-bedroom condominium at the Belnord priced at $4.995 million. The deal is slated to close in March.

Originally built in 1909, the limestone-and-brick building is listed on the National Register of Historic Places. Investors purchased the building for about $575 million in 2015 and converted more than half of the 211 apartments to condominiums, with most ranging in price from roughly $3.6 million to over $12 million, while almost 100 units are still rent-stabilized or rent-controlled apartments, according to Maya Kadouri, the Douglas Elliman agent representing the developer.

Roughly 65% of the converted condominiums have been sold since sales started in late 2018, representing about $300 million in sales, said Ms. Kadouri. About 10 units remain on the market, she said.

Ms. Kadouri said prospective buyers often mention the show on tours.

“A lot of people on tours say, ‘Is this the building from ‘Only Murders in the Building’? Or, ‘Is this the courtyard in the show?’” said Ms. Kadouri. She added, “the Belnord has a presence in the show that creates a desire to understand more about the building.”

Ms. Douglas, a writer who is moving to the Belnord from Rhode Island, said the show makes it seem like the Belnord has a sense of community. “I know it’s just a TV show, but seeing the different people they would encounter, it just made it feel like—for such a large apartment building—like a little bit of a neighborhood,” she said.

Still, the connection with the show doesn’t seem to materially affect potential buyers’ decisions, said brokers.

“At this price point,” Ms. Dreyfuss said, “no one is going to buy a condominium because of a show.”

Mr. Martin, who co-created “Only Murders” as well as starring in it, said he chose the Upper West Side as the setting because it was top of mind: He has owned a home at the San Remo on Central Park West for about 35 years.

“My mind just went to the Upper West Side because that is where I live,” said Mr. Martin. “I chose it by default.”

The Belnord’s courtyard and facade serve as the exterior of the Arconia on the show, which stars Mr. Martin, Selena Gomez and Martin Short as residents-turned-podcasting detectives. Interiors were shot on sets in the Bronx and at 270 Riverside Drive, another prewar building on the Upper West Side, according to season one location manager Collin Smith.

In addition to the Belnord, showrunners also considered the Apthorp on the Upper West Side and Harlem’s Graham Court as the primary filming location for the show, according to Mr. Smith. All three buildings have courtyards, which was critical for the plot, said co-creator John Hoffman.

“We knew we needed a sense of mystery, and the idea of a courtyard—it was a bit of a homage to ‘Rear Window,’” said Mr. Hoffman, referencing the 1954 movie starring Grace Kelly and James Stewart. “The idea that you can look out and see something happening in someone else’s apartment—we wanted that type of mystery around the place itself.”

Ultimately, the Belnord was selected, Mr. Smith said.

As for building residents, “the idea of filming in the courtyard was not warmly received initially,” said Julia Vitullo-Martin of the Belnord Landmark Conservancy, a group of building residents. But it helped that the crew offered positions as extras to any resident who wanted them. “I for one didn’t know that Selena Gomez was such a huge Disney star,” she said. “Her presence was a huge draw to youngsters in the building, who were very happy to be extras.”

Now, real-estate agents selling units in the building are making use of the show in their marketing.

To help advertise a $4.5 million listing at the Belnord, Brown Harris Stevens employee Arne Olson posted about the show on the company’s Instagram account in late October: “For $4.5M, you can live at the famed condominium…And if you haven’t watched @onlymurdershulu, you should!”

“The average consumer is not super into the real-estate market, so I hoped this post would make it a little bit more accessible,” said Mr. Olson, who had already watched the show before realizing BHS was listing a unit in the building. The unit is now under contract, though not in relation to the show, said listing agents Emma and Michael Kerins.

Even for agents marketing other Upper West Side buildings, “Only Murders in the Building” is helpful. “It’s a fun talking point to break the ice and make our showings more exciting,” said Douglas Elliman agent Michael Passaro, who recently brokered a $11,000-a-month rental in the Ansonia, a prewar apartment building on Broadway.

The Upper West Side luxury market is experiencing a strong rebound and had more than $271 million in sales in the fourth quarter of 2021, up 183% from 2020 and 217% from 2019, according to Jonathan Miller of Miller Samuel Real Estate Appraisers and Consultants. The Belnord had roughly $26 million in sales in the fourth quarter, almost quadruple the number in the same period of 2020 and almost three times as many as the fourth quarter of 2019, according to the real-estate data and analytics company Marketproof.

Ms. Kadouri attributed the Belnord’s transaction volume to the market’s recovery from Covid, and to the fact that the building’s courtyard provides outdoor space for residents. Plus, the building has six separate vertical sections, each with its own entrance, limiting interaction with neighbors and thus protecting against exposure to the virus, she said. “That became a more important aspect in buyers’ minds, given what they have gone through with the pandemic.”

Building residents, meanwhile, don’t seem to mind their home’s newfound fame. “There’s a nice feeling to it that makes us feel like we’re part of something,” said Belnord resident Amy Tonkonow Kohan, 62. She moved to the building to be closer to family, going into contract before she had heard of “Only Murders” and watching the show before closing. “It felt fun to see the courtyard especially,” she said.

Belnord doorman Eduardo Cruz, 61, who guards the courtyard from a booth outfitted in copper and brass, said he’s a fan of the show and that he feels honored to have the building featured, even if it means extra work keeping fans out of the courtyard. Nonresidents are generally not permitted to enter but can still take photos in front of the arch, he said.

“It’s mostly young girls who come and do all kinds of poses for the TikTok or who knows what,” said Mr. Cruz, who has been a doorman at the building for 33 years. “As long as they’re happy.”