The Japanese carmaker said it had suspended its global and Japanese sites early on Wednesday after they had been bombarded with traffic.
Hacktivists linked to the Anonymous collective have claimed responsibility. They tweeted the attack was "punishment" for Japan's killing of whales and dolphins.
Nissan has said that it has no view on such hunting activities.
Over recent weeks, Anonymous has struck against the websites of the Japanese president and other government departments in protest against the animals' deaths.
This follows related cyber-attacks against Icelandic institutions in November as part of the same OpWhales campaign, which resulted in most of the country's government sites being made unavailable for about 13 hours.
One of the hackers claiming responsibility for the attacks explained why Nissan had been targeted.
"They are a big corporation in Japan, and we have targeted big corporations to spread awareness about the killing [of dolphins] in the cove in Taiji because the Japanese news is censoring it," he said.
"As a note for Nissan, we are not out to harm your customer data or system data."
The attack coincides with the Detroit Auto Show, where Nissan unveiled a concept pickup van on Tuesday.
The company's US and European websites remain online.
"At Nissan, customer privacy and security is of utmost importance, and we take any potential threat to our information systems seriously," said a spokesman.
"Because of a potential distributed denial of service [DDoS] attack, we are temporarily suspending service on our websites to prevent further risks.
"Nissan continuously monitors and takes aggressive steps to ensure the protection of our information systems and all of our data."
Two of Nissan's main websites have been driven offline by a cyber-attack.