"The momentum is absolutely incredible," Cook said in a meeting with developers, government officials and journalists.
He said developers in China have earned over USD 7 billion, more than half of it in last one year.
Their apps are popular around the world, with many of them having been downloaded in hundreds of countries, Cook said.
"We are in the early phases of a tremendous growth," the 55-year-old CEO said.
Cook is on his eighth China visit since becoming Apple CEO in 2011.
He took a Didi taxi with Jean Liu Qing, president of Didi, to meet the developers.
The Apple chief regularly meets entrepreneurs when in China, and he said he continues to be impressed by them.
"[China] is one of the most vibrant places in the world. There are so many entrepreneurs now that they'll drive the next generation of innovation in China," he said.
He said the investment reflected Apple's excitement about Didi's fast-growing business and Apple's "continued confidence" in China's economy in the long term.
Cook did not give a direct answer when asked about rumours that the Didi investment has some connection with Apple's own plans for an electric car.
Currently, he said, Apple's focus when it comes to the car market is on CarPlay, it's device for connecting Apple smartphones to in-car entertainment systems.
But Cook became far more direct when asked whether the investment was a sign of Apple losing its innovative edge.
"No, no, there is no truth behind that at all," he said, arguing that it was a sign of Apple placing greater focus on China.
Apple has already put a lot of money into opening retail stores in China, which is Apple's second biggest market after the US.
It is expecting the 37th Apple Store to open this Saturday, approaching Cook's goal of having 40 before the end of 2016.
