Ashfield announced his illness in a letter to constituents on Friday.
"Over the past few weeks, I have not been feeling well and, while undergoing tests, doctors discovered that I have a form of cancer called Hodgkin’s lymphoma. We were fortunate to have caught this in the very early stages and, with treatment, there is a high rate of successful recovery," the fisheries minister said.
"My prognosis at this time is excellent. My treatment has begun and will continue through the summer."
Ashfield said he can carry out his duties until Prime Minister Stephen Harper shuffles his cabinet. At that point, Ashfield said he will focus on his recovery.
Harper said on Twitter that he and his wife, Laureen, are "saddened to hear" about Ashfield's cancer and "wish him well in his recovery."
"As always, the [Conservative Party of Canada] family is behind you," Harper said in a tweet.
New Brunswick Premier David Alward said he learned the news when speaking to Ashfield Thursday.
"I want to wish Keith the best as he battles this latest challenge and works toward a full recovery. He has been an able and dedicated representative of New Brunswick in Ottawa and is a valued former colleague in the provincial legislature," he said.
"Our thoughts and prayers are with Keith and his family and he takes on his most important fight to date. I look forward to his return to form and continued representation of our province."
The Fredericton MP also briefly stepped aside from his cabinet duties after suffering a heart attack last year.
The prime minister is expected to shuffle his cabinet in late June.
New Brunswick has two ministers sitting around Harper's cabinet table. Madawaska-Restigouche MP Bernard Valcourt is the minister of aboriginal affairs.
Ashfield has served in several portfolios for the Harper government, including National Revenue and the Atlantic Canada Opportunities Agency.
He also served as New Brunswick's minister of natural resources from 2003 to 2006.