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.
"We will work diligently to get service restored," said Timmins. "This was a service disruption issue. There was no compromise to customer data."
She says the bank has been cognizant of similar attacks on financial institutions elsewhere, particularly in the U.S.
"We've seen activity and you may be aware that unfortunately, it's frequently been the case in other countries, in the United States, that large financial institutions have been targeted," she said.
TD says it will be monitoring the situation closely in the coming days, and expects all of its services to resume in the next few hours.