"This is a victory brought by the people’s hope for overcoming crisis and economic recovery," she told supporters in the capital Seoul.
From the moment polls opened at 06:00 on Wednesday (21:00 GMT on Tuesday), millions of South Koreans queued to cast their ballots despite freezing temperatures.
Ms Park’s supporters cheered as poll figures emerged.
Both bolstered and dogged by the legacy of her father, who built South Korea’s economy while crushing dissent, she apologised in September for human rights abuses under his administration.
Mr Moon of the Democratic United Party is a former human rights lawyer who served under former President Roh Moo-hyun. He was briefly jailed by Ms Park’s father in the 1970s.