A review conducted by the agency has revealed over 700,000 people affected.
The agency said it was moving quickly to notify those people and would begin mailing notices by the end of February.
In a statement, IRS Commissioner John Koskinen said the agency was "committed to protecting taxpayers on multiple fronts".
The IRS said it found a further 295,000 taxpayer accounts were targeted, but not accessed by cyber criminals.
"Get Transcript" hack
In May, the IRS said it had uncovered criminals using personal information collected elsewhere, to access taxpayers "Get Transcript" application on the IRS website.
"Get Transcript" allows individuals to see information from previous years` tax returns. The application was shut down after the agency discovered the cyberattack.
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