The decision had been taken "for the future of our party and the country", said Hong Joon-Pyo, chairman of the Liberty Korea Party -- which has already changed its name from the Saenuri banner under which Park was elected.
Park was impeached by parliament in December after months of mass protests against her over a sprawling corruption scandal, and the constitutional court upheld the decision in March, removing her from office. She went on criminal trial in May.
Liberty Korea, which currently holds 107 seats in the 299-member parliament, is hoping that Park's expulsion will prompt some lawmakers to return and help it regain its status as the largest party from Moon's Democrats, who have 121 seats.
"I judged that in order for the Liberty Korea Party to regain its status as the main conservative bloc, it must cast off its yoke as the Park Geun-Hye party," Hong said.
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But the scandal that emerged last year unleashed nationwide fury, even among many of her conservative supporters.
Park denies multiple charges including bribery, coercion and abuse of power for offering governmental favours to tycoons, and is being held in custody.
It is unclear when the court will deliver a verdict on Park, whose trial was put on hold indefinitely after her entire defence team resigned to protest the court's decision to extend her detention.
Two former army-backed leaders who ruled in the 1980s and 1990s -- Chun Doo-Hwan and Roh Tae-Woo -- received lengthy prison sentences, but served only two years before being released under presidential pardons.
Hong described Park's fall from grace as "extremely regrettable" and signalled that he could pardon her if she was convicted and he won a future election.
The proceedings against her were an "extremely political trial", he said, adding: "Although former president Park Geun-Hye's party membership will be revoked today, I will do my utmost to make sure that she does not face any unfair punishment.