With a majority of 5-4, judges held that the 14th Amendment in the US Constitution, which regards equality before the law, enshrines the right of all to marry regardless of their sexuality.
Before, 13 of the 50 states had banned gay marriage at a federal level. The Supreme Court's ruling means they are now no longer allowed to do so and must legally conduct and recognise same-sex as well as heterosexual marriages.
Some say marriage is fundamentally for a man and women only, and that states should have the religious freedom to choose whether they allow or recognise same-sex unions or not.
Others say to withold marriage from gay people is discrimination, and that the institution is open to them as well.
Ronnie Floyd, the President of the Southern Baptist Convention, said in a statement: "I deeply believe in biblical and traditional marriage. The court has determined otherwise. Our #1 concern at this point is that religious freedom is protected in every way, honoring our God-given conscience, and that we not be discriminated against for our biblical and traditional stand.
"This decision shows one thing: Our desperate need for the next Great Awakening and the hope of the Gospel given to all persons. We must rise up like never before with great urgency, to forward the message of Jesus Christ to every person in America and across the world."
However Katharine Jefferts Schori, of The Episcopal Church, said in a statement: "I rejoice that the Supreme Court has opened the way for the love of two people to be recognized by all the states of this Union, and that the Court has recognized that it is this enduring, humble love that extends beyond the grave that is to be treasured by society wherever it exists.
"Our society will be enriched by the public recognition of such enduring faithful love in families headed by two men or two women as well as by a woman and a man. The children of this land will be stronger when they grow up in families that cannot be unmade by prejudice or discrimination. May love endure and flourish wherever it is to be found."
The United States Conference of Catholic Bishops said: "The unique meaning of marriage as the union of one man and one woman is inscribed in our bodies as male and female. The protection of this meaning is a critical dimension of the 'integral ecology' that Pope Francis has called us to promote."
"Mandating marriage redefinition across the country is a tragic error that harms the common good and most vulnerable among us, especially children. The law has a duty to support every child's basic right to be raised, where possible, by his or her married mother and father in a stable home."
And the US Presbyterian Church said on their website that it "... is celebrating the U.S. Supreme Court ruling that same-gender couples have a constitutional right to marry nationwide, striking down bans in 13 states. Church leaders believe today's ruling is a step in the right direction as society's views have continued to change in recent years.
"The PC(USA) has advocated for almost four decades for civil rights for lesbian, gay, bisexual and transgender persons. Today's decision by the U.S. Supreme Court is in keeping with that work."