A Hamptons insider was stunned to arrive at a house in Bridgehampton on Saturday night to find a packed party that looked “as if COVID had never happened” — with Donald Trump Jr. and girlfriend Kimberly Guilfoyle among the revelers.

Our spy estimated there were about 100 partiers, who our source says were maskless, at the bash at the 51 Sandpiper Lane mansion, hosted by famed Hamptons builder Joe Farrell, who owns the pricey pile and is selling one down the road for $15 million. The event — which comes days after Long Island entered Phase 3 of reopening and while COVID cases surge in some parts of the country — had a caterer and uniformed bartenders.

A source close to Don Jr. told us that the pair only stayed for about an hour, and that it was an outdoor event on the building’s roof (though our original source said that the party was both inside and outside).

Our source claims Ramona Singer also attended Saturday’s shindig. Neither she nor Farrell responded to our requests for comment.

Meanwhile, it seems that guests are being uncharacteristically cagey about the party — Don Jr., Guilfoyle, Farrell and Singer have all been mum on the party on their social media.