The company is now notifying 4.6 million users whose information was stored on a compromised database.
Scottrade also claimed client funds and its stock-trading platform were not affected.
"Client passwords remained fully encrypted at all times and we have not seen any indication of fraudulent activity as a result of this incident," Scottrade said.
The company is offering identity-protection services to potentially-affected customers.
Scottrade says it confirmed the breach after being alerted by federal authorities who were investigating cyber crimes targeting financial-services companies.
The revelations come a day after Experian, a credit-reporting agency, revealed it was the victim of a hack exposing the personal information of 15 million people who had applied for T-Mobile wireless services.