While other Republicans in the House have refused to endorse Republican nominee Donald Trump, so far none of the others have promised to cross the aisle and vote for his Democratic rival on November 8, US media reported.
Richard Hanna, who represents an eight-county district in upstate New York and is a former businessman, announced his decision in an interview and a letter on the opinion pages of local news site Syracuse.com.
"While I disagree with her on many issues, I will vote for Mrs Clinton," he wrote.
"Our response to the public's anger and the need to rebuild requires complex solutions, experience, knowledge and balance. Not bumper sticker slogans that pander to our disappointment, fear and hate," he added.
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Hanna said the former US secretary of state, first lady and New York senator had promoted many issues to which he has been committed, including the expansion of education and women's health care.
Syracuse.com reported that Hanna said in an interview he considered supporting Clinton for months, but decided to take action after watching Trump criticize the Muslim American parents of a US soldier killed in Iraq.
Hanna has served as congressman from New York since 2011 and previously announced plans to retire in December when his third term ends.
