Iowa's caucuses are the first nominating contests in presidential election cycles, giving the largely rural, Midwestern state an outsized role in choosing standard-bearers from each party.
"I just heard about it and I'm delighted," Warren told supporters after an event in Muscatine, Iowa. "It really means a lot to me."
Warren, the Register's choice, is a progressive who backs a single-payer healthcare system and reforms throughout the nation's economic, political and criminal justice systems.
In its endorsement, the newspaper said Warren would "push an unequal America in the right direction."
But the Register's editorial board went on to insist that Warren, viewed by some as too far left, is no radical.
"The senior U.S. senator from Massachusetts is not the radical some perceive her to be," the newspaper said. "She was a registered Republican until 1996. She is a capitalist."
In an Iowa poll released on Saturday by the New York Times, Warren came in third among Democratic voters, behind fellow progressive Vermont Senator Bernie Sanders and former Vice President Joe Biden. The Register's endorsement was the second high profile nod from a major newspaper this week. The New York Times endorsed her this week along with Minnesota Senator Amy Klobuchar, offering Warren for progressives and Klobuchar for those who prefer a more moderate approach.
In Muscatine, where Klobuchar was set to speak at a campaign event soon after an appearance by Warren, Dick Paul, 68, said he is leaning toward supporting Klobuchar because she has deep experience in government and "lots of good common sense."
"I don't pay much attention to endorsements," Paul said. "I don't think it will make a difference."
About 45 minutes after the announcement, Warren's campaign sent an email to supporters, sharing the news and asking for donations to help push turnout on caucus night.
"As exciting as these endorsements are, they won't mean anything if we don't do the work on the ground to turn out our supporters on February 3," the email said.
While Donald Trump, an incumbent who enjoys high favorability ratings within his party, is expected to handily win the Republican nomination, the Democratic field remains wide open, with five candidates leading a pack that started with more that two dozen contenders.
Top finishers in Iowa's nominating contests frequently go on to lead their parties in the final election match-up.