Bikar Atoll (Marshallese: Pikaar, [pʲiɡɑːrˠ]) is an uninhabited atoll in the Ratak Chain of the Marshall Islands. It is one of the smallest atolls in the Marshalls. Due to its relative isolation from the main islands in the group, Bikar's flora and fauna has been able to exist in a relatively pristine condition.

It is located 579 kilometers (360 mi) north of Majuro Atoll, the capital of the Marshall Islands, 320 kilometers (200 mi) South-southeast of Bokak, and 115 kilometers (71 mi) north of Utirik, the nearest inhabited atoll. The land area is 0.5 square kilometers (0.19 sq mi), surrounding a 37.4-square-kilometer (14.4 sq mi) lagoon. It consists of 6 islets.

The approximately diamond-shaped atoll measures up to 13 km north to south and up to 8 km across. Its six islets have a combined land area of less than 0.5 square kilometers (0.19 sq mi) and enclose a shallow lagoon of 37 square kilometers (14 sq mi). The surrounding reef is continuous except for one narrow pass located on the western side. The major islets are Bikar, Jabwelo, Almani and Jaboero. Bikar, the largest, reaches a height of 6 meters above sea level.

Based on the results of drilling operations on Enewetak (Eniwetok) Atoll, in the nearby Ralik Chain of the Marshall Islands, Bikar may include as much as 4600 feet of reef material atop a basalt rock base. As most local coral growth stops at about 150 feet below the ocean surface, such a massive stony coral base suggests a gradual isostatic subsidence of the underlying extinct volcano, which itself rises 10,000 feet from the surrounding ocean floor. Shallow water fossils taken from just above Enewetak's basalt base are dated to about 55 mya.

Low rainfall and high temperatures lead to arid conditions in which a freshwater Ghyben-Herzberg lens cannot form. The water at Bikar island is not as brackish as similarly arid Taongi Atoll, allowing coconuts planted by visiting islanders from Utirik Atoll to survive.

Like Taongi Atoll, the combination of an almost completely enclosed lagoon and waves driven over the reef by the prevailing northeastern trade winds results in the water level being perched some 0.5 m above the mean tide level.

Bikar is one of the driest of the Marshall Islands atolls, having a semi-arid character. Mean annual temperature is approximately 82 °F (28 °C). Mean annual rainfall is less than 45 inches (1,143 mm), and falls primarily during the July through October rainy season. Prevailing winds are north to north-easterlies.

Plant species of atoll forest include Pandanus tectorius, Lepturus repens, Cocos nucifera, Boerhavia repens, Pisonia grandis, Portulaca lutea, Triumfetta procumbens, Tournefortia argentea and Scaevola sericea, as well as areas of atoll scrub and vines. Much of the Pisonia forest noted in earlier surveys was devastated by a cyclone, perhaps Typhoon Mary in 1977. This is likely a naturally recurring event. A small number of coconut palms planted by visiting islanders survive. A 1952 survey found that they produced small nuts containing bitter milk.