"We are sorry about the downtime," wrote Koum. "It has been our longest and biggest outage in years. It was caused by a network router fault which cascaded into our servers."
"We worked with our service provider on resolving the issue and making sure it will not happen again."
WhatsApp was down for more than three hours on Saturday just days after Facebook bought it for $19 billion.
The five-year old company currently has about 450 million users worldwide and is the leading smartphone-based messaging app.