South Staffordshire PLC, the parent company of South Staffs Water and Cambridge Water, announced the attack via a statement.
South Staffs Water provides drinking water to approximately 1.3m people and has a head office in Walsall.
It insisted that the incident has not affected the supply of safe water and that customers will remain unaffected.
The statement added: "As you’d expect our number one priority is to continue to maintain safe public water supplies.
"This incident has not affected our ability to supply safe water and we can confirm we are still supplying safe water to all of our Cambridge Water and South Staffs Water customers.
"This is thanks to the robust systems and controls over water supply and quality we have in place at all times as well as the quick work of our teams to respond to this incident and implement the additional measures we have put in place on a precautionary basis.
"We are experiencing disruption to our corporate IT network and our teams are working to resolve this as quickly as possible. It is important to stress that our customer service teams are operating as usual.
"We are working closely with the relevant government and regulatory authorities and will keep them, as well as our customers, updated as our investigations continue."
The water provider added that as soon as the attack became apparent, immediate steps were taken to respond to and manage the incident and engaged leading forensic experts are investigating the issue.