The White House's site was down from 2 pm to 8 pm Eastern time on Tuesday due to the attack, said Jimmy Orr, a White House spokesman.
"Traffic was very heavy and blocked out most legitimate users," he said.
A DoS attack is one in which target servers are flooded with requests for information, often from multiple computers worldwide, thus overloading their ability to respond and blocking all access.
The site was not hacked, Orr said, and the White House IT staff was eventually able to bring the situation under control. The White House is working with the relevant federal agencies to share information about the attack, he said. The US Federal Bureau of Investigation and its Internet security arm, the National Infrastructure Protection Center, are usually involved in cases such as this.
Tuesday's attack was the third on the White House site in May, with the first two coming close to each other in the first week of May.
This week also saw a multi-day DoS attack against the federally-funded Internet security center CERT/CC. A study by researchers at the University of California at San Diego, also released this week, found that over 4,000 such attacks are launched every week.