The lights were spotted on at the former Next store at Ravenside Retail Park.
The retailer shut its Bexhill branch on August 7.
A Bexhill resident said: “I have been past the Next branch at Ravenside Retail Park since it closed and It looks like every single light in the store has been left on 24/7 and there must be hundreds in there.
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“One of the car park attendants said it’s been like that since it closed down and the store is completely empty. It must be costing them thousands a month in electricity bills.”
Green party councillor Polly Gray, who represents Bexhill Old Town & Worsham ward on Rother District Council, contacted Next to find out what was going on.
Next said its lease of the building ended in August when the branch closed and the landlord had been contacted to ensure the lights are switched off.
Cllr Gray said: “Lighting accounts for nearly five per cent of global CO2 emissions, and we are trying to reduce our carbon footprint in Rother.
“Switching off unnecessary lighting will help towards our aim of becoming carbon neutral by 2030.”
Next closed its Hastings store in Priory Meadow shopping centre in 2012.