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."
Five-time world player of the year Marta Vieira da Silva has joined Orlando Pride in America's National Women's Soccer League.