The organisation's code of conduct demands "abstinence before marriage and fidelity in marriage", but until now this only applied to heterosexual marriages.
World Vision is one of the largest Christian charities in the world, providing disaster and other forms of aid to 250 million people annually. It operates in nearly 100 countries worldwide and has revenue of approximately 1 billion US dollars per year.
Although globally World Vision hires more than 6,000 non-Christians in its 40,000 strong workforce, in the US it does not hire any. It has won court cases allowing it to discriminate on the basis of religion, and to require employees to sign a declaration of faith.
In a letter to all employees, World Vision USA President Rich Stearns explained that this latest decision was not motivated by theology: "I want to be clear that we have not endorsed same-sex marriage, but we have chosen to defer to the authority of local churches on this issue.
"World Vision is a multi-denominational organisation that welcomes employees from more than 50 denominations, and since a number of these denominations in recent years have sanctioned same-sex marriage for Christians."
World Vision's employees include many people from churches such as the Episcopal Church and the Evangelical Lutheran Church in America, both of which have chosen to bless same-sex marriages.
World Vision USA's headquarters are located in the state of Washington, which in 2012 legalised same-sex marriages after a referendum on the issue.
However, the policy change has received criticism from many significant Christian figures.
The president of the Southern Baptists' Ethics and Religious Liberty Commission, Russell Moore, spoke out against World Vision's policy on his website, where he claimed that: "At stake is the gospel of Jesus Christ.
"If sexual activity outside of a biblical definition of marriage is morally neutral, then, yes, we should avoid making an issue of it.
"If, though, what the Bible clearly teaches and what the church has held for 2000 years is true, then refusing to call for repentance is unspeakably cruel and, in fact, devilish.
"We empower darkness when we refuse to warn of judgement."
Criticising the possibility that the decision was made to broaden their appeal to the maximum possible number of potential donors, Mr Moore said: "Donor bases come and go. But the Gospel of Jesus Christ stands forever.
"World Vision is a good thing to have, unless the world is all you can see."
Pastor and theologian John Piper said on his website that World Vision's decision was: "A tragic development for the cause of Christ, because it trivialises perdition â€" and therefore, the cross.
"I pray they will repent and turn back to their more faithful roots."
Mr Piper said it was "fanciful" to interpret World Vision's decision as neutral on the issue of gay marriage.
Franklin Graham, the son of noted US evangelist Billy Graham, said he was "shocked" by World Vision's decision and described it as "ungodly" when he appeared on a Family Research Council radio programme.
Mr Graham, who is now president of the Christian international relief charity 'Samaritan's Purse' said: "It's obvious World Vision doesn't believe in the Bible."
In a statement on the Samaritan's Purse website, Mr Graham said: "World Vision maintains that their decision is based on unifying the church – which I find offensive – as if supporting sin and sinful behaviour can unite the church.
"From the Old Testament to the New Testament, the Scriptures consistently teach that marriage is between a man and woman and any other marriage relationship is sin."
Other Christians have praised World Vision's decision.
Denver based Pastor Nadia Bolz-Weber tweeted: "What a day. I think I'll start tomorrow by giving @WorldVisionUSA a big fat donation. Who's with me? I hear they are losing donors."
Matthew Skinner, associate professor of New Testament theology at Luther Seminary in St Paul, Minnesota, tweeted: "Happy to see @WorldVisionUSA take stand for Christian unity. Let's write checks to offset $ lost from those who prefer hard hearts."
Toby Jones, a theological author and professor from Fuller Theological Seminary said on Twitter that: "Evangelical leaders who are speaking out against @WorldVisionUSA get kudos in their echo chamber but lose a generation of seekers."
Mr Stearns has repeatedly insisted that this decision is not an example of World Vision's US branch taking sides on the issue.
In an interview with Christianity Today magazine in the US, Mr Stearns said: "This is not an endorsement of same-sex marriage. We have decided we are not going to get into that debate. Nor is this a rejection of traditional marriage, which we affirm and support.
"This is also not about compromising the authority of Scripture. People can say, 'Scripture is very clear on this issue,' and my answer is, 'Well, ask all the theologians and denominations that disagree with that statement."
In his 2010 book 'The Hole in our Gospel' Mr Stearns expressed his frustration and sadness over the fact that Christians are more often known for what they oppose rather than what they support.
"We're seen to be against homosexuality and gay marriage, against pornography and sexual promiscuity, against alcohol and drug abuse, abortion, divorce, Islam, evolution ... even against those who believe that global warming is a threat."
Mr Stearns instead argues that this action is a move to redirect focus away from issues relating to sexuality and "focus instead on uniting Christians around serving the poor".
Sarah Wilson, a spokesperson for World Vision UK told Christian Today that prospective employees on this side of the Atlantic do not have to disclose their sexuality.
She said: "It has never been our policy to ask our employees about their beliefs or their sexuality. There is no plans to change that policy, as the US branch's change brings their policy closer in line with ours.
"What Rich Stearns has been saying about the importance of the need to unite Christians around serving the poor is something we fully agree with and something we fully endorse."