TD Canada Trust says it was hit by a "targeted" cyber attack, forcing its banking website and mobile banking service to go offline for several hours.
The bank says the denial-of-service attack occurred mid-morning and prevented its customers from logging to its website and mobile site.
A denial-of-service attack occurs when hackers flood a website with a large amount of fake traffic, and in turn, preventing others access.
Bank spokeswoman Barbara Timmins says the attack had no effect on clients' personal information or accounts and that most of the services are now up and running.
She says business customers may still be unable to login to their online accounts.
The breach did not affect branch banking, telephone banking or ATMs.