Marissa Mayer is sitting on the hot seat — make that a white-hot seat.

The Yahoo! chief executive presided over a company “Great Gatsby” theme party this month on San Francisco’s Pier 48, the reported $7 million cost of which struck investors in the Internet also-ran as being tone-deaf to excess.

Mayer played the ultimate flapper — all in sequins on a “pure white armchair,” according to an account Tuesday on Vice’s Motherboard.

After being led through a series of velvet ropes, guests sat on a couch beside her for their commemorative photo before being dispatched to the party outside, where they indulged in Champagne poured by swinging aerialists.

The devastating details emerged a day after Eric Jackson, a managing director at SpringOwl Asset Management, dropped a 99-page slide deck calling for Mayer’s ouster, in which he blasted her for blowing money on everything from lavish parties to failed acquisitions.

A free iPhone for up to 22,000 employees? Check.

Gifts of Jawbone UP fitness wristbands to 11,000 co-workers close enough to Mayer to celebrate her first anniversary as CEO? Yep.

And don’t forget the free food for workers at Yahoo!’s Sunnyvale headquarters. That particular perk, according to Jackson, has already cost $450 million thus far into Mayer’s tenure.

“That’s half an Instagram right there,” Jackson said, referring to the $1 billion acquisition Yahoo! might have made instead of the $1.1 billion it spent on what he claims is a now-worthless Tumblr.

But the hedgie also let it be known Mayer wasn’t always so democratic when dispensing handouts. The Met Ball Gala, an A-list extravaganza organized by Vogue’s Anna Wintour, embraced Mayer last May as a co-host. But the $3 million Yahoo! paid to sponsor the event generated tickets only for its CEO and two lieutenants.

A similarly elite group of execs accompanied Mayer, via NetJets, to Davos. Yahoo! sponsored that gathering, too, spending $1 million to $2 million, he estimates.

“What was the business case for the Met and Davos?” he asked.