She had been expected to declare her candidacy for months
Mrs Clinton ran for the Democratic presidential nomination in 2008 but lost to Barack Obama.
`Listening tour`
A few hours into her journey from New York, Mrs Clinton tweeted: "Road trip! Loaded the van & set off for IA. Met a great family when we stopped this afternoon. Many more to come. -H."
Mrs Clinton`s team said she would spend the next few weeks building up grassroots support in the early Democratic primary states.
Her first rally, to officially kick off her campaign, is not expected until mid-May. But her trip to Iowa is to be a "listening tour" where Mrs Clinton will meet voters at low-key events.
Later this week, she is expected to meet groups of students, teachers and small business owners.
On Sunday, Mrs Clinton launched her campaign website and declared in a video that she was running for president.
"Americans have fought their way back from tough economic times," she said, "but the deck is still stacked in favour of those at the top.
She added that she wanted to be a champion for "everyday Americans".
The video features a number of Americans talking about their hopes and aspirations.
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