Judges unanimously upheld Parliament's decision to impeach her over her role in a corruption scandal involving close friend Choi Soon-sil.
Ms Park's dismissal from office means South Korea must now elect a new president by early May.
She also loses her presidential immunity and can be prosecuted.
Ms Park had been suspended from presidential duties since December, with the country's prime minister taking over her responsibilities
Ms Choi meanwhile has been charged with bribery and corruption for allegedly pressuring big companies to give money in return for government favours. Ms Park has been accused of colluding with her.
Both women have denied wrongdoing.
'Impaired spirit of democracy'
A panel of eight judges with the country's apex constitutional court had deliberated Ms Park's impeachment before deciding there was sound reason for her dismissal.
The court said she leaked many documents in breach of a rule on securing official secrets, and violated the law by allowing Choi to meddle in state affairs, according to news agency Yonhap.
Her action had "seriously impaired the spirit of... democracy and the rule of law," said constitutional court chief justice Lee Jung-Mi. "President Park Geun-Hye... has been dismissed."