Barbara Thomas went hiking with her husband in California's Mojave Desert on Friday and has been missing since, police said.
Thomas, 69, and her husband were separated during the hike, according to the San Bernardino Sheriff's Department.
"Barbara was last seen wearing a black bikini, a red baseball cap and tan hiking boots with black socks," the department said. She's a resident of Bullhead City, Arizona, and has blonde hair and green eyes, authorities said.
She doesn't have any supplies or a cell phone with her, police said.
Temperatures have climbed to over 100 degrees since authorities began their search.