Brooklyn Beckham's billionaire father-in-law, Nelson Peltz, just broke his silence on Brooklyn's nasty family drama ... and he says Brooklyn needs to keep the details off the internet.

Nicola Peltz's pops was asked Tuesday during a Q&A session at the WSJ Invest Live event in West Palm Beach, FL about Brooklyn airing his family's dirty laundry in public and he revealed what he told his family on how to deal with such issues.

Nelson said ... "My advice is to stay the hell out of the press."

He also made light of the situation, joking ... "Has my family been in the press lately? I haven't noticed at all."

Brooklyn's father-in-law went on to say Nicola and Brooklyn are great and he looks forward to them "having a long, happy marriage together."

As you know ... Brooklyn went scorched earth last month, calling out his parents in a series of social media posts ... dropping bombshell claim after bombshell claim and calling David and Victoria controlling, fake, and straight-up dishonest.

Seems Nelson would have preferred Brooklyn to keep it all in the family.