David Geffen is firing back at his estranged husband … he says the guy’s claims that Geffen promised to financially support him for life are bogus and a textbook shakedown attempt.

The billionaire just responded to a lawsuit filed by his estranged husband, David Andrew Armstrong (AKA Donovan Michaels) … and Geffen flat out denies ever making assurances to take care of Armstrong financially for the rest of his life.

In the docs, obtained by TMZ, Geffen says Armstrong’s claims are ludicrous and part of a contrived attempt to humiliate Geffen in public and extract a financial settlement.

Geffen says it’s clearly a shakedown but insists it won’t work. He says during their relationship, he was happy to provide Armstrong with anything he asked for, putting him up in his own apartment in New York and giving him “nearly unlimited spending money.”

But Geffen says he never promised to share ownership of any assets with Armstrong or provide him with life-long financial support if they ever separated.

TMZ broke the story … Geffen and Armstrong did not have a prenup when Geffen filed for divorce in May after less than 2 years of marriage. Forbes says Geffen is worth $8.8B but he’s been retired for 15 years, and our understanding is he has no income other than investments, including property sales and dividends from stocks and equities he mostly acquired long before he met Armstrong. Any assets he got before they married would be Geffen’s separate property.

These docs are a response to the separate civil lawsuit Armstrong filed against Geffen in July, claiming Geffen promised him they would share all earnings and accumulated property equally. He also claimed Geffen told him he would financially support him for life.

Geffen says his written estate plan makes it clear that was never his intention … and flat out insists there were no such promises made to Armstrong.

Geffen’s attorney Patty Glaser filed the response … and we’re told Geffen’s divorce attorney, disso queen Laura Wasser, brought Patty on for the civil case.

In the docs, Geffen says Armstrong was treated like a king when they were together and Armstrong used his money to buy expensive clothes, get cosmetic procedures, and go on extravagant trips with his friends … taking advantage of Geffen’s “good nature and generosity.”

Geffen claims Armstrong spent hundreds of thousands of dollars on OnlyFans subscriptions and male prostitutes towards the end of their marriage … and says the guy was happy to spend Geffen’s money on himself and his friends, while only feigning interest in Geffen.

What’s more … Geffen says Armstrong was hiding illicit drug use from him — regularly using ketamine and cocaine — and claims the drug habit continued even after Geffen paid for him to get treatment.

Geffen says he ended the relationship as Armstrong’s “facade of a loving, devoted husband began to crumble” and he discovered the extent of his ex’s “betrayal, dishonesty and outright fraud.” He says he was faced with the reality that the man he loved was not a true partner but had used him for a life of excess shared with others … and decided to end things.