Phetchaburi is located at the northern end of the Malay Peninsula, with the Gulf of Thailand to the East and the Tanaosi mountain range forming the boundary to Myanmar. Except these border mountains most of the province is a flat plain area. With an area of about 3000 km² the Kaeng Krachan National Park is Thailands largest national park, covering nearly half of the province. It protects mostly rainforests in the mountains along the boundary to Myanmar, but also the Kaeng Krachan reservoir is part of the park. The only significant river of the province is the Phetchaburi River.
Several caves are found in the province, most notable the Khao Luang caves near the capital city. Illuminated by a hole in the roof several Buddha statues are placed in the cave.