The two girls were "touched inappropriately" during a visit to the attraction, in Windsor, Berkshire, on 11 August, police said.
The man, in his 20s from Bracknell, had been bailed to a specified date but has since been released without charge.
Thames Valley Police have released an e-fit of the offender.
The assaults happened between 12:15 BST and 12:30 at Castaway Camp in the Pirate Shores area of the attraction, the force said.
Det Insp Penny Mackenzie said she believed the offender travelled to Legoland on the day of the offence with others - either family or friends.
"We are working closely with Legoland Windsor to progress this investigation and this includes a review of the CCTV.
"About 14,200 people attended the theme park that day so this will be a painstaking process and one that needs to be carried out thoroughly and carefully.
"If you recognise the face of the man in the E-Fit or the description given, please contact police immediately", she added.
A man arrested on suspicion of sexually assaulting two six-year-olds at the Legoland theme park, has been released without charge.