Four years ago, Madonna was selling out shows at Madison Square Garden. Now, she’s struggling to sell out the BAM opera house in Brooklyn.

The “Material Girl” singer’s “Madame X” tour that kicks off in New York City on Sept. 12 is off to a surprisingly sluggish start, with hundreds of seats still available for most gigs after more than a month on the block, The Post has learned.

That’s despite the fact that the 60-year-old pop star’s upcoming tour will focus on small, theater-type venues that typically only have a few thousand seats each.

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After initial reports that the first shows sold out, an interactive seat map that went live on Ticketmaster’s website this week shows that hundreds of seats are still available for each gig.

For an Oct. 7 concert at the 2,100-seat BAM Howard Gilman Opera House — the last night of a 17-night stand that’s slated for Brooklyn — more than half of its $357 mezzanine seats were unsold as of Tuesday afternoon, according to Ticketmaster’s map.

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Prime seats directly in front of the stage in the orchestra, also priced at $357, were still available by the dozens. Meanwhile, only half of the $257 balcony seats were taken, with many already being offered at a discount on StubHub — one as low as $220.

That shows a “shocking” lack of interest in Madonna, according to one insider. Her 2015-2016 “Rebel Heart Tour” grossed $170 million at venues that often drew more than 30,000 fans for a single performance.

On that tour, she reportedly sold out Madison Square Garden twice. In 2012, she was said to sell out Yankee Stadium in 20 minutes.

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Madonna’s “Madame X” tour is being promoted by Live Nation, which insisted in a statement that a huge number of seats are now available because it has “increased the number of shows exponentially due to overwhelming demand.”

“We added 10 shows in New York alone (a total of 17) to accommodate Madonna’s vast fanbase,” the concert promoter told The Post. “We removed suspicious requests throughout the campaign to ensure as many real fans as possible were able to access tickets. With the first round of tickets completely fulfilled and delivered to verified registered fans, Live Nation released a limited amount of additional tickets yesterday (Monday).”

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Nevertheless, all 17 shows at the BAM opera house showed at least 150 seats still available, including opening night on Sept. 12.

After New York, the tour will move to the 3,600-seat Chicago Theatre in October, and then onto gigs including the 4,300-seat Colosseum at Caesars Palace in Las Vegas; the 1,800-seat Wiltern Theatre in Los Angeles and the 3,500-capacity Wang Theatre in Boston.

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At several of those dates, nearly half of the tour’s tickets and VIP packages remain unsold — despite lottery sales since Madonna announced on May 6 that her “intimate” gig would take place mostly in 2,000-seat venues.

“It’s very telling these shows didn’t sell out as quickly as ‘Springsteen on Broadway’ did a couple of years ago,” an insider said. “This clearly isn’t the home run Team Madonna was hoping for.”

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Her “Madame X” album, due out on June 14, could add to the momentum, although early signs aren’t encouraging. None of the five songs released before the record itself drops has debuted on the Billboard Hot 100.

This article originally appeared on Page Six.