Katherine Megan McArthur is an American oceanographer and a NASA astronaut.

She was born in 1971 in Honolulu, Hawaii but later in her childhood moved to California. She went to St. Francis High in Mountain View, California, then later graduated from UCLA in 1993 with a degree in Aerospace Engineering; in 2002, she earned a Ph.D. in oceanography from Scripps Institution of Oceanography at UC San Diego.

At the Scripps Institution of Oceanography, McArthur conducted graduate research in nearshore underwater acoustic propagation and digital signal processing. Her research focused on determining geoacoustic models to describe very shallow water waveguides using measured transmission loss data in a genetic algorithm inversion technique. She served as Chief Scientist during at-sea data collection operations, and has planned and led diving operations during sea-floor instrument deployments and sediment-sample collections. While at Scripps, she participated in a range of in-water instrument testing, deployment, maintenance, and recovery, and collection of marine plants, animals, and sediment. During this time, McArthur also volunteered at the Birch Aquarium at Scripps, conducting educational demonstrations for the public from inside a 70,000 gallon exhibit tank of the California Kelp Forest.