The retail banking website of JP Morgan Chase, the largest US bank by assets, was hit by a denial-of-service attack on 12 March that took some of its services temporarily offline, preventing customers from logging on to their accounts.
The website was interrupted for around 90 minutes, from 5-6.30pm EST.
A denial-of-service attack occurs when attackers bombard websites with on overwhelming amount of traffic to overload servers, which reduces performance or cripples the service.
The bank said on its Twitter feed that its mobile banking applications were not affected and that its branches were available for service.
Security experts have yet to precisely determine where the attacks originated from.
A spokesman confirmed that no customer data had been compromised.
The incident follows a similar outage in February.
Cyber assaults on banks have increased in recent months, with attacks on Bank of America, Wells Fargo and Citigroup.