Disgraced upstate Republican Rep. Chris Collins resigned from Congress on Monday — a day before he was expected to plead guilty in Manhattan federal court to insider trading charges.

The Buffalo-area lawmaker submitted a letter of resignation to House Speaker Nancy Pelosi on Monday, officials there confirmed to The Post.

It will take effect tomorrow.

Collins also presented a letter of resignation to New York Governor Andrew Cuomo, in which he states he’s immediately stepping down from office.

“I hereby submit my resignation effective immediately, September 30, 2019, as United States Representative of the 27th District of New York,” reads the letter, which is written on embossed congressional letterhead.

On Tuesday, he will enter a change of plea at a 3 p.m. hearing, bringing an end to the year-long saga, according to a Monday court filing and courthouse sources.

The congressman’s son, Cameron Collins, and co-defendant Stephen Zarsky were expected to cop to unspecified charges on Thursday.

The case centers around an Australian biotech company, Immunotherapeutics, where the lawmaker served as a member of its board of directors.

Prosecutors claim Collins called his son with an inside stock tip after the company failed a key drug trial.

Cameron allegedly dumped his stock and passed the news to Zarsky, leading him to sell off his shares too.

The trio faced a slew of fraud, wire fraud, securities fraud and related charges.

Lawyers for the Collinses did not respond to requests for comment Monday. Zarsky’s attorney declined to comment.

–Additional reporting by Emily Saul