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.
Subsequent messages suggested that she needed money for private medical care and asked for the money to be transferred directly.
The correct spelling of her unusual name helped convince the family it was genuine, and her grandfather agreed to pay the supposed bill.
Attempts to contact her directly failed, as the calls went straight to an answerphone.
Only after the money was paid did they get through to her, and realised they had been tricked.
"You feel such a fool," her grandfather said. "I was angry that I was able to be duped.
"You get used to these scam calls, but they are getting quite clever. I used to run my own business, so if they can fool people like me, a lot of very vulnerable people will be in trouble."
He is trying to get the money refunded from his bank, but so far they have said their fraud checks were sufficient and have refused to reimburse him.