This is the Pope's second visit to South America
Ecuador's left-wing President Rafael Correa called the visit "an honour".
Pope Francis will also travel to Bolivia and Paraguay, as part of his second trip to the region since becoming pontiff in 2013.
The Argentine is the first Pope to come from South America. He is expected to highlight the themes of poverty and inequality.
In 2007, before becoming Pope, he told a gathering of Latin American bishops that they were living in the most unequal part of the world.
During his previous trip in 2013, he addressed millions on Brazil's Copacabana Beach.
The Pope has requested to chew coca leaves while in Bolivia, according to Bolivian Culture Minister Marko Machicao.
Coca, the raw ingredient for cocaine, has been used in the Andes for thousands of years to combat altitude sickness and as a mild stimulant.
In September the Pope will travel to Cuba ahead of a trip to the US.
The pontiff is credited with helping bring about last December's diplomatic thaw between the two countries.
