Twitter's information security director Bob Lord said about 250,000 users' passwords had been stolen, as well as usernames, emails and other data.
Affected users have had passwords invalidated and have been sent emails informing them.
Mr Lord said the attack "was not the work of amateurs".
He said it appeared similar to recent attacks on the New York Times and the Wall Street Journal.
The US newspapers reported that their computer systems had been breached by China-based hackers.