Karen LuJean Nyberg (born 7 October 1969) is an American mechanical engineer and NASA astronaut. Nyberg is the 50th woman in space.

Born on October 7, 1969 in Parkers Prairie, Minnesota, Nyberg graduated summa cum laude with a degree in mechanical engineering from the University of North Dakota in 1994. She continued her collegiate studies at the University of Texas at Austin. There her graduate research was centered on human thermoregulation and experimental metabolic testing and control, focusing on the control of thermal neutrality in space suits. This work, performed at the Austin BioHeat Transfer Laboratory, led to her doctorate in 1998.

She worked at Johnson Space Center from 1991 to 1995 and received a patent for work she completed in 1991 on Robot Friendly Probe and Socket Assembly. In 1998, on completing her doctorate, she accepted a position with the Crew and Thermal Systems Division, working as an Environmental Control Systems Engineer to improve space suit thermal control systems and evaluate firefighter suit cooling technologies. She also provided conceptual designs of the thermal control system for the Advanced Mars and Lunar Lander Mission studies, and environmental control system analysis for a collapsible hyperbaric chamber.