The footballer was allegedly taken by surprise when reporter Elena Di Cioccio put on a pair of rubber gloves and grabbed his private parts.
"I want to find out how big his testicles are!" ContactMusic quotes Di Cioccio as saying.
Beckham, who was speaking to press outside a Milan hotel, apparently recoiled in shock and was led away by security staff.
Di Cioccio then reportedly ran down the street shouting, "E piccolo, Beckham", which translates as "Beckham is small".
The journalist is said to have wanted to see whether Beckham's package was as impressive as it appears in his Armani underwear campaign.
She later told Italia1: "Mmm, my God. I touched his b***s."