The 31-year-old Brazil forward, better known just by her first name, has signed a two-year contract with an option of a third.
She has recently been playing in Sweden for FC Rosengard.
Marta was part of the Brazil side which lost in the 2007 World Cup final to Germany in China.
Marta won five consecutive world player of the year titles from 2006-2010 and has finished in the top three of the voting in every year from 2004-2014.
Orlando City chief executive Alex Leitao said: "We are incredibly proud to bring Marta to Orlando, and I am personally very excited as I have been a fan of her for many years.
"Our technical team worked hard to add the best, most accomplished women's soccer player in the world to the roster."

Firefighters dealt with blazes on Friday overnight at garage forecourts in Salisbury and Durrington as well as in residential areas of Amesbury.
An aircraft was also found burnt out at Old Sarum Airfield. Police estimate the damage caused at "over £100,000".
A man, 26, from Amesbury was arrested on Saturday on suspicion of arson.
All of the "suspicious fires" took place between 01:38 GMT on Friday and 04:40 GMT on Saturday, Wiltshire Police said.
Det Con Ian Day said: "All the vehicles were unoccupied at the time of the fires and no one was injured however the damage caused is significant and is likely to run over £100,000.
"We are treating the fires as suspicious and believe that the vehicles were deliberately set alight.
"We have made an arrest but I would like to appeal for any witnesses to come forward as they may be able to assist our investigation."

Speaking to Huw Edwards on The Wales Report, Andrew RT Davies defended his party from criticism that it simply opposes Labour policies rather than putting forward its own programme for government.
"At every juncture we bring forward an alternative, we don't just criticise," he said.
"There is a clear agenda for what the Welsh Conservatives stand for, and it's action.
"It's action to improve the economy, it's action to improve public services in Wales and it's action to strengthen communities the length and breadth of Wales."
Asked about his priority if the Tories won power after the assembly election in May, Mr Davies said: "The first spending commitment is protecting the NHS budget for the lifetime of this parliament, or the assembly's term, the five years.
"I would suggest that the outcomes in Wales regrettably aren't as good as we want them to be.
"And we want to make sure those outcomes are improved.
"That's why we've called for an independent Keogh-style enquiry into the NHS here in Wales so we can make those improvements, not on political whims, but led by clinicians themselves telling us what we need to be doing to improve the health service here in Wales."
Prof Sir Bruce Keogh investigated 14 NHS trusts in England for a review into higher-than-expected hospital death rates, published in 2013.
Speaking about the Wales Bill on further devolution, the Welsh Conservative leader praised Welsh Secretary Stephen Crabb's handling of the legislation, put on hold on Monday after MPs called for a re-think.
It followed claims that the draft bill was confusing, and would leave Welsh ministers with fewer powers rather than more.
Mr Davies said: "The secretary of state deserves huge credit; for instead of ploughing on with this bill, actually saying 'I've listened to what people have said to me'.
"Actually, the Welsh Labour government could learn a lot from this legislative process."
First Minister Carwyn Jones had called Monday's announcement on the Wales Bill an "avoidable delay to clear up an avoidable mess".
Five-time world player of the year Marta Vieira da Silva has joined Orlando Pride in America's National Women's Soccer League.
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A man has been arrested following a spate of suspected arson attacks on dozens of cars and an aircraft, at nine different locations in south Wiltshire.
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Labour has been "ploughing on as if they have a divine right to rule" in Wales, the Welsh Conservative leader has said.