The Chilean Constitutional Court on Monday issued its 14-page ruling that upheld the measure. The judges reached their decision on April 2.
A group of opposition lawmakers challenged the bill after it received final approval in the Chilean Congress in January.
President Michelle Bachelet, who is a member of the Socialist Party of Chile that is part of a coalition of center-left political parties known as the New Majority that governs the country, supported the bill.
Rolando Jímenez, president of the Movement for Homosexual Integration and Liberation, a Chilean LGBT advocacy group, on Twitter urged Bachelet to sign the measure into law now that it has been upheld as constitutional.
Fundación Iguales, another Chilean LGBT advocacy group, described the ruling as “excellent.”
Por fin el ! Tribunal Constitucional determino Constitucionalidad de Ley Union Civil esperamos que Gobierno lo promulgue rápidamente !
— Rolando Jiménez (@rjimenez_perez) April 6, 2015
— Fundación Iguales (@IgualesChile) April 6, 2015
Neighboring Argentina is among the countries in which gays and lesbian couples are able to legally marry.
Bachelet — who publicly supports nuptials for gays and lesbians — in February announced that her government will not oppose marriage rights for same-sex couples in a lawsuit the Movement for Homosexual Integration and Liberation filed in 2012 on behalf of three gay couples.
The case remains before the Inter-American Commission on Human Rights, which is based in D.C.