Typhoon Ketsana, known in the Philippines as Tropical Storm Ondoy, was the second-most devastating tropical cyclone of the 2009 Pacific typhoon season, causing $1.15 billion in damages and 921 fatalities, only behind Morakot earlier in the season, which caused 789 deaths and damages worth $6.2 billion. Ketsana was the sixteenth tropical storm, and the eighth typhoon of the season. It was the most devastating tropical cyclone to hit Manila, surpassing Typhoon Patsy (Yoling) in 1970.

Ketsana formed early about 860 km (530 mi) to the northwest of Palau on September 23, 2009. The depression remained weak and was downgraded to a low pressure area later that day by the Japan Meteorological Agency (JMA) and after drifting through extremely favorable conditions, it intensified the next day and was categorized as Tropical Depression by the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) and was given the name Ondoy after entering the Philippine Area of Responsibility. The Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert on the depression. It was then upgraded to a tropical depression by the JMA later that morning before the JTWC followed suit early on September 25, designating the depression as 17W. Soon, Ketsana was upgraded to a tropical storm before passing over the Philippines. As it moved into the South China Sea the storm intensified while moving toward the west, and was categorized as a Severe Tropical Storm by the JMA.

President Gloria Macapagal Arroyo declared a "state of calamity" encompassing most of Luzon after at least 86 people were initially reported dead in landslides and other incidents. Flood water levels reached a record 20 feet (6.1 m) in rural areas. As of October 24, 2013, at least 464 deaths in the Philippines were officially reported from the typhoon.