The 2009 Pacific typhoon season has no official bounds; it runs year-round in 2009, but most tropical cyclones tend to form in the northwestern Pacific Ocean between May and November. These dates conventionally delimit the period of each year when most tropical cyclones form in the northwestern Pacific Ocean. The scope of this article is limited to the Pacific Ocean, north of the equator and west of the International Date Line.

Storms that form east of the date line and north of the equator are called hurricanes; see 2009 Pacific hurricane season. Tropical storms formed in the entire Western North Pacific basin are assigned a name by the Japan Meteorological Agency. Tropical depressions formed in this basin are given a number with a "W" suffix by the United States' Joint Typhoon Warning Center.

In addition, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) assigns names to tropical cyclones (including tropical depressions) that enter or form in the Philippine area of responsibility. These names, however, are not in common use outside of the Philippines. Since the 2000 season, the Laboratory for Atmospheric Research at the City University of Hong Kong has forecast the expected number of tropical cyclones, named storms, and typhoons in a season.

Forecasts are released in April and June. This season, the CityUHK is predicting an about average season. An average season, according to the CityUHK, has 31 tropical cyclones, 27 named storms, and 17 typhoons.

In its April forecast, the CityUHK predicted 31 total tropical cyclones, 27 named storms, and 18 typhoons. It also forecast that 4 tropical cyclones would make landfall in Southern China, all of which are expected to be in the early season (between May and August). A normal season sees 5 such landfalls, of which 3 are early and two are late (September to December).

On June 15, PAGASA reported that seven to ten tropical cyclones would move through their area of responsibility during the next 3 months.

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