The two girls were "touched inappropriately" during a visit to the attraction, in Windsor, Berkshire, last Thursday, police said.
A 22-year-man, from Bracknell, has been arrested on suspicion of sexual assault and is currently in custody.
Thames Valley Police previously described the offences as "extremely rare".
Detectives have appealed for any witnesses to come forward.
The assaults happened between 12:15 BST and 12:30 at Castaway Camp in the Pirate Shores area of the attraction, the force said.
A Legoland spokesperson said the parks' management were "deeply shocked" by the incident.
A man has been arrested on suspicion of sexually assaulting two six-year-olds at the Legoland theme park.