The 75-year-old, who wished to remain anonymous, said they had been tricked by criminals posing as his grand-daughter on the messaging service.
He transferred £1,550 to the con-artists, for an emergency medical bill that was a fake.
WhatsApp and trading standards officers are warning others of the scam.
'You feel a fool'
Fraudsters posing as the young student sent a message to her father, saying she had a case of haemorrhoids that she was embarrassed to talk about.