Mauna Loa Solar Observatory (MLSO) is a solar observatory located on the slopes of Mauna Loa on the island of Hawaii in the U.S. state of Hawaii. It is operated by the High Altitude Observatory (HAO), a laboratory within the National Center for Atmospheric Research (NCAR). The MLSO sits on property managed by the Mauna Loa Observatory (MLO), which is part of the U.S. Department of Commerce National Oceanic and Atmospheric Administration (NOAA). MLSO was built in 1965.

Mauna Loa is one of five volcanoes that form the Island of Hawaii in the U.S. state of Hawaii in the Pacific Ocean. Mauna Loa is Earth's largest active volcano by both mass and volume. It was historically considered to be the largest volcano on Earth until Tamu Massif was discovered to be larger. Mauna Loa is a shield volcano with relatively gentle slopes, and a volume estimated at 18,000 cubic miles (75,000 km3), although its peak is about 125 feet (38 m) lower than that of its neighbor, Mauna Kea. Lava eruptions from Mauna Loa are silica-poor and very fluid, and tend to be non-explosive.