Kumarsinghe, who holds the citizenship of Sri Lanka and Switzerland, had moved the apex court against a lower court's ruling which had disqualified her in May.
The 61-year-old United People's Freedom Alliance (UPFA) Member of Parliament was elected from southern district of Galle in 2015 general elections.
She was also a member of Rajapaksa's joint opposition group which challenged President Maithripala Sirisena's leadership in the Sri Lanka Freedom Party (SLFP).
The SLFP and Prime Minister Ranil Wickremesimnghe's United National Party (UNP) are coalition partners in the present unity government. The Rajapaksa backers had been demanding Sirisena to fire Wickremesinghe to form their own SLFP government.
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The 19th Amendment, which became law just a few months ahead of parliamentary elections, bars a person holding dual nationality from being a candidate.
Kumarsinghe was allowed to contest the elections by returning officers under the 1981 Parliamentary Elections Act.
Holding that the provisions of 19th Amendment was applicable in the elections, the Court of Appeal overturned the returning officer's decision and unseated Kumarasinghe on the ground that she was an unsuitable candidate at the time of nomination.