The Internal Revenue Service said on Thursday that it planned a hiring spree this tax season as it tries to clear a backlog of more than 20 million unprocessed tax returns from previous years.

The agency said it intended to hire 10,000 new employees in the next year and hoped to bring on about half of those workers in the next few months. The I.R.S. described the blueprint as an “all hands on deck” approach to emerge from longstanding staffing shortages that were compounded by the pandemic.

“To ensure inventory is back to a healthy level for next filing season, we are leaving no stone unturned — taking an all-hands-on-deck approach to ensure as many employees as possible are dedicating time to return processing,” Charles P. Rettig, the I.R.S. commissioner, said in a statement.

This tax season has been the most challenging in the history of the I.R.S., current and former employees say. Longstanding budget cuts and staff shortages collided with the pandemic, as the agency became the main distributor of stimulus relief, including direct checks and tax credits.

The I.R.S. has suffered from tight budgets for decades, as Republicans sought to starve the agency. It received more than $1 billion in additional funding through the American Rescue Plan last year, though, to improve taxpayer services. A 2022 House spending bill that passed this week would give $12.6 billion to the I.R.S., the largest annual budget increase since 2001.

The hiring goals will not necessarily be easy to accomplish, as the nation has been experiencing a labor shortage and many of the I.R.S. jobs pay $15 an hour, making them less competitive than many private sector jobs.

The I.R.S. expects to hold job fairs in Kansas City, Mo.; Austin, Texas; and Ogden, Utah, to beef up its processing centers in those cities. It is also shifting 700 employees to those locations to create “surge teams” to process backlogged returns.

Officials from the Treasury Department, which oversees the I.R.S., said on Thursday that the goal was to have the inventory of old tax returns cleared by next year’s tax season. There are no plans, they said, to delay Tax Day this year beyond April 18.