Company officials suspect Chinese hackers of breaking into the newspaper's servers to gain access to employee user names and passwords.
They said they learned of the attack Wednesday after Mandiant, a cybersecurity contractor, noticed the intrusion.
"This is an ongoing investigation, but we believe it was a few days at most," Post spokeswoman Kris Coratti said.
The hackers didn't access subscriber information, such as credit cards and addresses. They also didn't hack into the newspaper's publishing system, emails or personnel information.
The newspaper asked its employees to change their user names and passwords on the assumption that many or all had been compromised.