The data could include Social Security numbers and employment history.
Gap has said that it is offering free credit monitoring services and fraud resolution assistance for the next year to all those affected.
"We are reviewing the facts and circumstances that led to this incident, and will take appropriate steps to help prevent something like this happening again, " said chief executive Glenn Murphy.
Gap claimed that not all applicants are affected as the company uses more than one vendor to manage its job applicant data, and that Canadian applicants' Social Insurance numbers were not among the details stolen.
Calum Macleod, European director at security firm Cyber-Ark, argued that the data should have been encrypted.
"It clearly wasn't, so now the Social Security numbers, employment records and a wealth of other information on 800,000 people who applied for jobs at Gap, Old Navy, Banana Republic and Outlet stores across the US, are potentially available to identity thieves," he said.
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iTnews Executive Retreat - Security Leaders Edition
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