The city of Muang Na Rang was reconstructed in 1845, after it had been abandoned during the fall of the Ayutthaya kingdom in 1767. The town was rebuilt at the mouth of the Ron River and renamed Prachuap Khiri Khan, which means the city in the mountains.
King Mongkut gathered the people of 3 towns—Bang Nangrom, Kui Buri and Khlong Wan—and had them settle the rebuilt city. At the same time, he renamed Koh Kong, a city on the opposite side of Gulf of Thailand, Prachanta Khiri Khet. Koh Kong is a now a province of Cambodia.
In 1868, King Mongkut invited foreign guests to watch a solar eclipse on September 18. He had predicted the event by himself, but the chosen observation point was at the marshes near Sam Roi Yot. He contracted malaria, which killed him just two weeks later.
Prachuap became a seaside resort during the reign of King Rama V. In the reign of King Rama II, a new city was established at the mouth of the I Rom Canal known as Muang Bang Nang Rom.
At the beginning of World War II, Japanese troops invaded Thailand. On December 8, 1941, they struck first near the city of Prachuap Khiri Khan. After fighting valiantly in battle, the defenders were ordered to ceasefire by the government in Bangkok and decision to allied with Japan until the end of World War II.