STORY: Huge areas of the country are inundated, and hundreds of thousands of people have been forced from their homes.
The government says nearly 33 million lives have been disrupted.
Pakistan estimates the damage at $30 billion, and both the government and Guterres have blamed the flooding on climate change.
The U.N. secretary-general landed in Sindh province on Saturday, before flying over some of the worst-affected areas en route to Balochistan, another badly hit province.
"Pakistan needs massive financial support. This is not a matter of generosity, it is a matter of justice," Guterres said after landing in Sindh, according to a video released by the office of Prime Minister Shehbaz Sharif.
A video released by Information Minister Marriyum Aurangzeb showed Guterres seated next to Sharif viewing flood-damaged areas from an aircraft window.
Guterres added on Saturday the world needed to understand the impact of climate change on low-income countries.