In its first global report on the infection, the WHO said that with millions at risk of a slow progression to chronic liver disease, cancer and premature death, swift action on testing and treatment was needed.
"Viral hepatitis is now a major public health challenge that requires an urgent response," the WHO's director general Margaret Chan said in a statement.
The 325 million cases reported are of hepatitis B (HBV) or hepatitis C virus (HCV) – the main types of the five different hepatitis infections and responsible for 96 percent of deaths from the disease.
HBV infection requires lifelong treatment, for which the WHO recommends tenofovir, a generic anti-viral drug also used in HIV treatment.
Hepatitis C can be cured relatively swiftly, but the medicines are too expensive for many patients.
Pressure over pricing has been growing, notably on U.S. manufacturer Gilead Sciences - which has developed some of the most effective treatments - and the company has taken some steps to offer discounts and provide access programs.
That includes allowing Indian drugmakers to manufacture much lower-cost versions of them for sale in developing countries.
Gottfried Hirnschall, director of WHO's Department of HIV and the Global Hepatitis Programme, said the WHO was working with governments, drugmakers and diagnostics companies to improve access.
"More countries are making hepatitis services available for people in need - a diagnostic test costs less than $1 and the cure for hepatitis C can be below $200," he said. "But the data clearly highlight the urgency with which we must address the remaining gaps in testing and treatment."
Viral hepatitis killed 1.34 million people in 2015, a toll comparable to tuberculosis and HIV/AIDS. But while TB and AIDS deaths are falling, hepatitis deaths are on the rise and have increased by 22 percent since 2000, the WHO said.
Around 1.75 million people were newly infected with HCV in 2015, bringing the global total to 71 million, with experts identifying unsafe healthcare procedures and injection drug use as the top causes.
New B virus infections are falling, thanks to a vaccine given as a part of childhood immunization that 84 percent of babies born in 2015 were given, according to the WHO report.