The 23-month-old boy was found unresponsive at an address in Nottingham Road, Somercotes at around 1.30pm on Wednesday, Derbyshire Police said.
The youngster was taken to the Royal Derby Hospital but died despite the best efforts of medical staff.
In a statement, the force said: “Officers were called to reports that a 23-month-old boy had been found unresponsive in a paddling pool in Nottingham Road, Somercotes at around 1.30pm on June 2.
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“The boy was taken to the Royal Derby Hospital, however, despite the best efforts of staff he sadly passed away.
“Our thoughts are with his family at this most difficult of times.
“Officers are continuing their inquiries into the circumstances around the incident on behalf of the coroner.”
East Midlands Ambulance Service (EMAS) said paramedics were called to a commercial address at 1.23pm.
In a statement, and EMAS spokeswoman said: “We received a call at 1.23pm on June 2 to a commercial address in Somercotes.
“The caller reported a medical emergency.
“We sent two paramedics in fast response cars and a double-crewed ambulance.
“The air ambulance was also in attendance.
“One patient was taken to Royal Derby Hospital via double-crewed ambulance.”