Sources told Fox News the former House speaker's resignation is expected to become effective in December or January, although no date had been set for a formal announcement. Hastert had said in August he planned to leave Congress but at that time didn't disclose whether it would be before the end of his term.
His early departure will mean a special election in his Illinois district, which he has represented for 11 terms.
Hastert followed Newt Gingrich as House speaker in 1999, losing the post to Rep. Nancy Pelosi, D-Calif., when the Democrats took control this year. He is one of 12 Republican lawmakers to announce their retirements this year.