The boy, whose identity has not yet been released, was declared dead at an Aurora hospital on Sunday night after rescue divers pulled him from Jericho Lake in west suburban Montgomery.
He was at the park with his parents, siblings and other relatives when he apparently went swimming with his siblings, according to Fox Valley Park District police. At some point, his siblings lost sight of him and did not know if he had returned to the shore or was still in the water, authorities said. The boy was reported missing just after 6 p.m.
Divers from Aurora and Sugar Grove searched the waters for about 45 minutes until they located the boy submerged in about 12 feet of water about 25 feet from the shore, officials said.
The boy was pulled from the water and rushed to Provena Mercy Medical Center, where he was pronounced dead, officials said.
Jericho Lake Park features a 20-acre lake for boating and fishing, but swimming is strictly prohibited and signage is prominently posted, park district Executive Director Nancy McCaul said in a statement.
Officials with the Kane County coroner's office have not returned calls seeking comment.