SkyWest flight 5971, a scheduled domestic passenger flight from Aspen/Pitkin County Airport (ASE), Aspen, Colorado, to George Bush Intercontinental Airport (IAH), Houston, Texas, diverted to Austin-Bergstrom International Airport (AUS), Austin, Texas, after encountering severe turbulence at FL390. As a result, one passenger received serious injuries. The first officer, the pilot flying, stated they did not see significant weather near their route from ASE to IAH, but the flight crew did brief on the possibility of small thunderstorm cells developing as a threat while flying toward IAH. The flight crew stated that extra fuel was added to the airplane in preparation for possible deviations due to weather and that flight attendants (FAs) were briefed on turbulence when departing ASE. The departure from ASE was without incident. While en route to IAH, the flight crew was cleared by air traffic control (ATC) to deviate left and right of the planned flight course due to thunderstorms in the area. The flight crew stated the seatbelt sign was illuminated due to these deviations from the planned flight course. The captain stated they used the onboard weather radar to look for heavy precipitation returns, but none showed up for their flight trajectory. Furthermore, the captain stated they did not receive information from their dispatcher or recommendations from ATC prior to encountering severe turbulence. The flight crew recalled that the encounter with severe turbulence was sudden and unanticipated when the flight was clear of clouds. The first officer performed an upset recovery procedure and began an immediate descent and the captain contacted ATC and declared an emergency. The airplane leveled off at FL370 at the conclusion of the upset recovery procedure. When the severe turbulence was encountered, one flight attendant (FA) was in the aisle preparing the cabin and the other was in the forward galley. Both FAs were thrown to the floor and remained there until the turbulence subsided. There were also two passengers out of their seats at the time of the turbulence. One passenger was in the aisle and was thrown to floor but was able to return to his seat. The other passenger was discovered by the aft FA in the aft lavatory with head injuries. The FA assisted this passenger to the aft FA jumpseat. Shortly after the recovery, the flight crew received a “cabin altitude high” warning on the engine indication and crew alerting system (EICAS). The flight crew donned their oxygen masks and initiated an emergency descent to 10,000 ft. Subsequently, the flight diverted to AUS and landed without further incident. Emergency medical personnel met the airplane at the gate and transported both passengers to the hospital. One passenger was treated and released with minor injuries and the other was diagnosed with a fractures to her vertebra, tibia, and ribs. Postaccident examination of the airplane found the outflow valve that regulates cabin pressure had malfunctioned.