The IRS had been updating more than 50 systems used to process tax returns, and the shutdown, which ended last week, has delayed the process.
"Updating these core systems is a complex, year-round process with the majority of the work beginning in the fall of each year," the IRS said.
Filing season, previously scheduled to begin Jan. 21, will now start no earlier than Jan. 28 and no later than Feb. 4, the IRS said in a statement.
"Readying our systems to handle the tax season is an intricate, detailed process and we must take the time to get it right," said acting IRS Commissioner Daniel Werfel.
The April 15 filing deadline remains the same under federal law.
This is the second year in a row the IRS has delayed the start of tax season. Earlier this year, filing was delayed until Jan. 30 because Congress made last-minute changes to tax law.