Judges unanimously upheld parliament’s decision to impeach Ms Park over her role in a corruption scandal involving her close friend, Choi Soon-sil.
She now loses her presidential immunity and could face criminal charges.
There have been angry scenes outside the court. Police said two protesters had died.
The court ruling is the culmination of months of political turmoil and public protest. An election must now be held within 60 days.
Ms Park’s office said she would not be leaving the Blue House, South Korea’s presidential palace, on Friday nor making any statement.
Acting President Hwang Kyo-ahn has called for calm, saying the government should remain stable to prevent internal conflict from spreading.
At the heart of the drama lies the close friendship between the president and Ms Choi.
Ms Choi is accused of using her presidential connections to pressure companies to give millions of dollars in donations to non-profit foundations she controlled.
Ms Park is alleged to have been personally involved in this, and to have given Ms Choi unacceptable levels of access to official documents.
Parliament voted to impeach Ms Park in December and the Constitutional Court has since been deciding whether to uphold or overturn this.
On Friday, a panel of eight judges ruled Ms Park’s actions “seriously impaired the spirit of… democracy and the rule of law”.
The court said she had broken the law by allowing Ms Choi to meddle in state affairs, and had breached guidelines on official secrets by leaking numerous documents.
Ms Park had “concealed completely Choi’s meddling in state affairs and denied it whenever suspicions over the act emerged and even criticised those who raised the suspicions,” it said.
But the judges dismissed some charges, including accusations Ms Park had infringed on freedom of the press by creating a media blacklist of cultural figures, and criticism of her response during the 2014 Sewol ferry disaster.
Ms Park was already suspended from presidential duties, with the prime minister taking over her responsibilities.
But she must now leave office – and her official residence – and a presidential election will be held within the next 60 days.
She has also lost her presidential immunity so could now face criminal charges over allegations she colluded with Ms Choi.