The Federal Trade Commission is cracking down on “influencers” — pop stars, athletes and musicians who promote products on social media without disclosing that they are being paid for their endorsements.

The FTC said Wednesday it had sent some 90 warning letters to influencers and marketers reminding them that they must “clearly and conspicuously disclose their relationship to brands” when paid to promote them on Instagram or other social media.

It marked the first time the FTC has specifically warned the celebrities themselves.

The warnings come in response to a complaint filed last September by Public Citizen and three affiliated watchdog organizations that asked the federal agency to investigate ties between 113 influencers and their products for possible noncompliance with disclosure regulations.

“The ‘influencer’ industry on Instagram represents one of the most prominent and ethically egregious violations of FTC policy,” said the complaint filed by the watchdog groups.

Cosmetics and beauty companies appeared to be among the worst offenders, Public Citizen said in its complaint, but it also questioned fashion houses, footwear makers, alcoholic beverages and energy drinks.

The targets of the complaint included such boldface names as Kim, Khloe and Kourtney Kardashian, Rihanna, Victoria Beckham, Vanessa Hudgens, Lindsay Lohan and Jenny McCarthy.

The FTC would not identify the specific celebrities or the marketers who had received the warning letters.

“In sending the letters, the staff did not predetermine in every instance whether the brand mention was in fact sponsored as opposed to an ­organic mention,” the FTC said.

But if paid, the celebrities were warned that as endorsers they must disclose to consumers that the thumbs-up amount to an advertisement.

Guys also got in on the act, according to Public Citizen, which questioned product mentions by Michael Phelps, LeBron James and Louis Tomlinson.

All the examples were pulled from Instagram accounts, although Public Citizen said it could not determine if all of the examples in its complaint were paid endorsements.