The 74-year-old Democrat issued a statement on Thursday morning to his constituents advising that he has "joined the ranks of millions of Americans" while describing what steps he plans to take moving forward.
Connolly has been in office since 2009, with Tuesday's win over Republican Mike Van Meter representing the beginning of his ninth term.
"It was a surprise because, except for some intermittent abdominal aches and pains, I had no symptoms," he wrote. "I'm going to undergo chemotherapy and immunotherapy right away.
"Cancer can be tough. But so am I. I will attack this the only way I know how - with Irish fight and humor."
Senators Mark Warner and Tim Kaine issued a statement of their own on Thursday after Connolly made his announcement, calling him "a fighter - plain and simple."
"Gerry has always been a steadfast advocate for the people of Northern Virginia, and we know that he will continue to show that same fierce commitment as he faces this battle.
"We are confident he will emerge from this even stronger, as he has time and time again."
Connolly said that he is "confident in a successful outcome," and cautioned he may be "fatigued" as he undergoes treatment and continues duties in office.
"Over the next few months as I do my job here in our district and on the Hill, I may be a bit fatigued due to the treatment. I hope you understand," he said.
"Be assured I'm full of hope and am sustained by the love, support and encouragement of my family, friends, and of all of you," Connolly continued. "I look forward to serving you all in this new term and in the future."
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