Thailand has hundreds of islands in the Gulf of Thailand and the Andaman Sea, but most people only know Phuket, Koh Samui, and maybe Koh Phi Phi. Here are six Thai islands you may never have heard of.

Koh Kret is a true hidden gem. Although it lies only 30 km upstream in the Chao Phraya River, making it the closest island to Bangkok, most tourists are unaware of it. The island is dotted with temples, known as "Wats." The most impressive of these is Wat Poramaiyikawat, whose dilapidated, whitewashed Mutao Pagoda is Koh Kret's most striking landmark. The island is particularly recommended on weekends, when a market selling food and handicrafts springs up along a narrow path through the busiest part of the island.

Laoliang is a truly remote island national park with a stunning beach that – surprisingly – is neither overrun with longtail boats nor tourists, and it seems likely to stay that way. A maximum of 40 people are allowed to stay overnight in tents at any one time; day visitors are not permitted. There are no bungalows or resorts either. The two karst islands of North and South Laoliang, rising from the Andaman Sea 20 kilometers off the coast, are an hour's journey by longtail boat from Hat Samran Pier in Trang.

Koh Phayam is often described as "the Koh Samui of the 1970s"—a step back in time in the Andaman Sea. And while resorts like the CEDE Boutique Beach Resort offer barefoot luxury, the island remains relatively untouched. Rent a kayak and paddle along the south coast. Discover Aow Khao Kwai (Buffalo Bay), a beach with lagoon-like waters, dazzling white sand, and small bars and restaurants tucked away in rocky, forest-fringed coves.

Koh Tarutao is located in Tarutao National Park on the southern Andaman coast. The archipelago is surrounded by dense rainforests, towering limestone cliffs (the highest measuring over 700 meters), and white sand beaches. The main island is a 45-minute boat ride from Pak Bara Port. There are no modern tourist amenities like ATMs or noisy jet skis. Accommodation options include bungalows and campsites. Due to its remote location, the island once served as a prison—Thailand's version of Devil's Island—and was even a filming location for a season of the television show "Survivor."

The Similan Islands National Park is only open from November to May and, being about an hour's speedboat ride from the mainland, remains relatively untouched. Khao Lak, north of Phuket, is the closest gateway from the mainland. During the season, day trips depart daily, and numerous liveaboard dive boats are available for longer safaris. The diving and snorkeling are spectacular: colorful hard and soft corals and an incredible variety of marine life await you.

Just a short boat ride from the popular resort town of Krabi lies the little-known island of Koh Klang, a tranquil little Muslim enclave. Although only five minutes by longtail boat from Krabi Town, it feels like another world. Learn to fish with the locals and kayak through the lush mangrove forest and around the towering limestone islands before enjoying lunch at one of the floating restaurants and savoring the day's catch.