Typhoon Lan, known in the Philippines as Super Typhoon Paolo, was the third-most intense tropical cyclone worldwide in 2017, behind only hurricanes Irma and Maria in the Atlantic. A very large storm, Lan was the twenty-first tropical storm and ninth typhoon of the annual typhoon season. It originated from a tropical disturbance that the United States Naval Research Laboratory had begun tracking near Chuuk on October 11. Slowly consolidating, it developed into a tropical storm on October 15, and intensified into a typhoon on October 17. It expanded in size and turned northward on October 18, although the typhoon struggled to intensify for two days. On October 20, Lan grew into a very large typhoon and rapidly intensified, due to favorable conditions, with a large well-defined eye, reaching peak intensity as a "super typhoon" with 1-minute sustained winds of 249 km/h (155 mph) – a high-end Category 4-equivalent storm – late on the same day. Afterward, encroaching dry air and shear caused the cyclone to begin weakening and turn extratropical, before it struck Japan on October 23 as a weaker typhoon. Later that day, it became fully extratropical before it was absorbed by a larger storm shortly afterward.

The 2017 Pacific typhoon season was a below-average season in terms of accumulated cyclone energy and the number of typhoons and super typhoons, and the first since the 1977 season to not produce a Category 5-equivalent typhoon on the Saffir–Simpson scale. The season produced a total of 27 named storms, 11 typhoons, and only two super typhoons, making it an average season in terms of storm numbers. It was an event in the annual cycle of tropical cyclone formation, in which tropical cyclones form in the western Pacific Ocean. The season runs throughout 2017, though most tropical cyclones typically develop between May and October. The season's first named storm, Muifa, developed on April 25, while the season's last named storm, Tembin, dissipated on December 26. This season also featured the latest occurrence of the first typhoon of the year since 1998, with Noru reaching this intensity on July 23.