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The city of Prachuap Khiri Khan was reconstructed in 1845, after it was abandoned during the fall of the Ayutthaya kingdom in 1767. The town was rebuilt at the mouth of the Ron River during the 19th century and renamed Prachuap Khiri Khan.
King Mongkut gathered 3 cities -- Bang Nangrom, Kui Buri and Khlong Wan -- to settle and named the area Prachuap Khiri Khan province. At the same time, he renamed Koh Kong, the city that located on the east side of Gulf of Siam(recently Gulf of Thailand), as Prachanta Khiri Khet. Now Koh Kong is a province of Kingdom of Cambodia.
In 1868, King Mongkut invited several guests to watch the solar eclipse on September 18. Being fond of science he predicted the event by himself, but the chosen observation point was the marshes near Sam Roi Yot where he contracted malaria, of which he died two weeks later.
It became an old seaside resort of the country during the reign of King Rama V. From historical evidence, Prachuap Khiri Khan was a location of Mueang Na Rang during the Ayutthaya period. In the reign of King Rama II of Rattanakosin, a new city was established at the mouth of the I Rom Canal and was named Mueang Bang Nang Rom. Moreover, during the reign of King Rama IV, Mueang Bang Nang Rom, Mueang Kui, and Mueang Khlong Wan were combined into Mueang Prachuap Khiri Khan which means a city of mountains. The city hall was situated at Mueang Kui until 1898, it was then moved to Ao Ko Lak or Ao Prachuap, the location of Mueang Prachuap Khiri Khan nowadays.
During World War II, Japanese troops occupied Thailand. On December 8, 1941, they first landed near the city of Prachuap Khiri Khan. After one day of battle the Thai troops resigned and had to allow Japan to use Thailand as a base for their war operations.