The six-year-old went missing on Saturday afternoon after he fell into the River Stour in Sandwich, Kent, while on a fishing trip with his family.
The body has not yet been formally identified but his family have been informed, Kent Police said on Wednesday morning.
Inquiries are continuing but the death is not being treated as suspicious, a force spokesman said.
Chief Inspector Mark Weller said: "This is a deeply tragic and upsetting incident for all concerned, and Lucas' family are understandably devastated.
"We were inundated with offers of assistance from members of the public, who turned out in large numbers to help search for Lucas.
"I know his family are very grateful and touched by the support provided, and I too would like to offer my own personal thanks."
He also thanked emergency services for their efforts.