So POTUS, FLOTUS, the VP and the Second Gentleman all walk into a bar … much to the delight of a bunch of stunned patrons.

President Biden and his wife, Jill — flanked by Kamala Harris, her husband, Doug, and some Secret Service agents, of course — hit up the popular D.C. restaurant, Le Diplomate … in a Memorial Day visit the White House says was unscheduled.

As you can see … the diners weren’t expecting such famous guests to be joining them for lunch, but everyone looked happy as heck about it.

Earlier Monday, Biden and Harris and their spouses attended a wreath-laying ceremony at Arlington National Cemetery’s Tomb of the Unknown Soldier.

Biden’s Le Diplomate appearance is his first known sit-down visit to a restaurant as Prez, coming not long after the CDC lifted COVID-19 restrictions for those who are vaccinated.

BTW — the French eatery is a known favorite of the First Lady’s … it even added her favorite dish as a menu special earlier this year after Biden was elected President.