Emergency services were called at around 3pm after receiving reports that the young boy had got into difficulty in the water.
Multiple emergency services rushed to the scene including specialist search teams in a desperate bid to find him.
Police have confirmed that the search team discovered a body in the reservoir but no formal identification has taken place yet.
A spokesperson said: “We are sorry to share that the underwater search team has found a body in Ulley Reservoir in Rotherham.
"No formal identification has taken place. The boy’s parents have been informed and are being supported by specialist officers.”
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