Mayor Bill de Blasio’s spending on his “special assistants” ballooned by $4.6 million over the past fiscal year as he fattened their ranks by nearly 13 percent and doled out raises to those already on staff, The Post has learned.

The bloated taxpayer-funded roster of de Blasio’s vaguely titled aides — who are accountable only to Hizzoner and constitute about 40 percent of his already large staff — grew by another 34 to 298, payroll records show.

The move helped swell their collective salaries to $23.3 million during the year that ended June 30, up nearly 25 percent from fiscal 2016, according to payroll records.

Those paid more than $100,000 each jumped to 84, up about 30 percent, while four pocketed more than $200,000 each.

De Blasio also shifted four special assistants to the Department of Veterans Services, offloading nearly $540,000 in spending from the Mayor’s Office.

Among the dozen highest-paid special assistants, two — senior adviser Andrea Hagelgans and press secretary Eric Phillips, who made $217,084 and $193,135, respectively — are responsible for helping to burnish the mayor’s image.

A third, special counsel Henry Berger, who made $198,816, is the mayor’s longtime election lawyer.

At least three of the special assistants who were on the public payroll during fiscal 2017 have since left to work on de Blasio’s November re-election bid.

A former city official said bestowing the title of “special assistant” on his underlings gives de Blasio increased flexibility to shower them with the public’s hard-earned cash.

“When you want to give someone a big raise, it’s easier if you say they’re doing a different job,” the source said.

Among de Blasio’s highest-paid special assistants, chief of staff Kevin O’Brien — who was paid $198,342 in fiscal 2017 — saw his annual salary skyrocket 26 percent, from $175,000 to $220,652, since fiscal 2016, records show.

The added cost of the special assistants is part of an overall 16 percent hike in spending on Mayor’s Office personnel, which rose from $46 million to $53.4 million, records show.

When The Post first revealed last year how the mayor had boosted the number of special assistants by 140 percent over predecessor Mike Bloomberg, de Blasio retaliated by ignoring a Post reporter at a news conference and publicly attacking the paper as a “right-wing rag.”

The temper tantrum earned him widespread condemnation from other media outlets — along with an instantly classic Post front page that showed him pitching a fit like an angry baby in a highchair.

Likely Republican challenger Nicole Malliotakis called de Blasio’s spending on his special assistants “unconscionable.”

“It just shows a complete disregard for the taxpayers of the city and it also displays his mismanagement — because we’re not even getting results,” said Malliotakis.

A City Hall spokesman said the use of the title gives the administration “more flexibility in demanding varied types of work from senior officials.”