Emergency services were called to reports of a fire at a four-storey building on Sandbed Street in Kilmarnock at around 9pm on Monday.
Six fire engines and two specialist units battled the blaze at its height, and crews worked through the night to bring the incident under control.
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There were no reported injuries.
Police confirmed on Tuesday that they were treating the fire as wilful and that an investigation had been launched.
A 12-year-old boy has now been arrested and charged in connection with the incident.
He is due to appear at Kilmarnock Sheriff Court later this month.
A statement from Police Scotland said: “A 12-year-old male has been arrested and charged in connection with a fire in Kilmarnock.
“Around 9.30pm on Monday, 14 July, 2025, we received a report of a fire at a premises on Sandbed Street.
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“There were no reported injuries.
“The youth has been released on an undertaking to appear at Kilmarnock Sheriff Court later this month.
“The fire is being treated as wilful and enquiries are ongoing.”
Anyone with any information is asked to contact Police Scotland on 101 quoting reference 3692 of July 14.
