The Massachusetts Democrat made the announcement Friday. It was expected that she would run again.
Warren has emerged as one of Republican President-elect Donald Trump's fiercest critics in Congress.
She said in the message that Massachusetts voters did not send her to Washington to "roll over and play dead" while Trump and his appointees "crush the working people of our Commonwealth and this country."
Warren was recently named to the Senate Armed Services Committee.
Among the Republicans who could challenge Warren in 2018 is former Boston Red Sox pitcher Curt Schilling, a strong Trump backer.
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