Phahonyothin Road (, , ) or Highway 1 is a main road in Bangkok and one of the four primary highways in Thailand, which include Mittraphap Road (Highway 2), Sukhumvit Road (Highway 3), and Phet Kasem Road (Highway 4). It begins at Victory Monument in Bangkok and runs north to the Burmese border, with a total length of .
Following the Boworadet Rebellion in October 1933, the government of Phraya Phahonphonphayuhasena proposed the construction of a highway linking central Bangkok with Don Mueang Airfield to strengthen its control over the facility. Construction began in March 1934 and was completed in July of the same year. The road was originally named "Prachathipat Road" (, Thanon Prachathipat), spanning a distance of .
In 1938, Prime Minister Plaek Phibunsongkhram ordered the extension of the road from Don Mueang through Bang Pa-in, Ayutthaya, Saraburi, Lopburi, and Sing Buri, bringing its total length to . The extended road was renamed Phahonyothin Road in honor of Phraya Phahonphonphayuhasena (personal name Phot Phahonyothin), the second prime minister of Thailand and one of the leaders of the 1932 Siamese revolution.
In Bangkok, Phahonyothin Road originates at the northeast corner of Victory Monument on the periphery of Ratchathewi and Phaya Thai districts, then crosses Chatuchak, Bang Khen, Don Mueang, and Sai Mai districts before continuing into Pathum Thani Province, and on through Ayutthaya, Saraburi, Lopburi, Nakhon Sawan, Chainat, again through Nakhon Sawan, Kamphaeng Phet, Tak, Lampang, Phayao and finally Chiang Rai, ending in Mae Sai district, where it connects to Tachileik in Myanmar.
Phaholyothin Road in Bangkok is also a demarcation line between districts or sub-districts in several phases as follows: