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."

Nancy Soderberg said the Northern Ireland peace process was a large part of Mr Clinton's legacy as US president.
"This is something he cares very deeply about, and if there's a way he can help, of course he would be willing to," she told the BBC's Sunday News.
Northern Ireland's political parties have been holding round-table talks.
It follows a row sparked by a police assessment that IRA members were involved in murdering a former IRA man.
In the wake of the killing of Kevin McGuigan Sr, police said the IRA still existed, but added that it was not engaged in terrorism.
Three Democratic Unionist Party (DUP) ministers subsequently resigned from Northern Ireland's ruling executive, and Peter Robinson, the party's leader, stepped aside as first minister.
Mr Clinton's offer of assistance was made public on Saturday by Irish prime minister Enda Kenny, who met him last week in New York.
Ms Soderberg said the ultimate responsibility for achieving progress lay with the Northern Ireland parties.
In an Irish Times article last year, she accused Northern Ireland politicians of an "abysmal abdication of leadership" and being "stuck in the past".
Asked if this was still her view, she said: "The fact that you need to call back Bill Clinton to move things forward is evidence that this remains true.
"That's not to say that this is easy - I don't mean to sit over here on this side of the pond and just throw darts, but after all that Washington has invested in this, as well as London and Dublin, the fact that they're still questioning whether you can have a devolved government is quite stunning."
She added: "It should give the leaders pause to say 'what's missing in this equation?'
"My view is it's trust - this generation doesn't trust itself and what it's not doing is cultivating the next generation that will."
Northern Ireland Secretary Theresa Villiers said on Thursday that this week's talks had been "useful and intensive".
Ms Villiers said the talks would resume on Monday to "discuss the impact of continued paramilitary activity".
Two of Nissan's main websites have been driven offline by a cyber-attack.
[NEXT_CONCEPT]
Bill Clinton has a genuine willingness to help Northern Ireland parties resolve the crisis at Stormont, a former senior aide has said.