Bank of America Corp. says its online banking service was down for about four hours but service has been restored.
A representative for America's largest bank, Tara Burke, declined to specify a reason for Friday's outage except to say that it was a "temporary system" issue.
She could not say whether the site has experienced a similar across-the-board outage before.
The bank, based in Charlotte, North Carolina, said service was restored at around 5:15 p.m. EDT (2115 GMT). The outage began at around 1:25 p.m. EDT (1715 GMT). Some customers may still have trouble signing on because of the volume of people trying to access the site.
Customers can also get account information from ATMs or banking centers. Burke said none of Bank of America's 18,000 ATMs were affected by the outage.