Bachelet, 62, would take office March 11 to succeed conservative billionaire President Sebastian Pinera. She had served as Chile's first woman president back in 2006.
"It is clear at this point. She won. And we congratulate her. Later on, I will go speak with her personally," rival Evelyn Matthei told reporters.
The national electoral board said Bachelet earned 62.59 percent of the vote against Matthei's 37.40 percent, with 56 percent of votes tallied.
Bachelet's contest with Matthei marked the first time in Latin America that a presidential runoff was held between two women.
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In the first round, which resulted with Bachelet winning 47 percent of the vote to 25 percent for Matthei, more than 50 percent of voters did not bother to cast ballots.
Bachelet has focused on greater social justice in a country that has the highest per capita income of any Latin American country.
She has proposed increasing taxes to raise $8.2 billion for the state coffers. She wants everyone, not just the rich, to have access to free post-secondary education.
She plans to bring Chile in line with a wave of social liberalism sweeping once conservative Latin America, which include legalizing abortion and opening discussions on same-sex marriage.
During her first term, Bachelet reformed the pension system, improved health and social services, and focused on the well-being of Chile's working class and elderly.
Her presidency coincided wit a boom in global demand for copper, Chile's top export.