This incident occurred when Horizon flight 2184 landed on a parallel taxiway to runway 6 at Pullman/Moscow Regional Airport (PUW), Pullman, Washington. Pullman/Moscow Airport was not served by an air traffic control tower (i.e. an uncontrolled airport). The runway lighting at KPUW was not operating at the time of the incident and there was no NOTAM regarding the inoperative lights.  However, at the time of landing, the taxiway lights on the parallel taxiway were operational. The lighting system had been tested about two hours prior to the incident landing but airport personnel did not notice the runway lights were not functioning until informed by the flight crew.  Following the incident landing, airport personnel found that the runway lighting wiring vaults were flooded, likely from recent heavy rains and snow from the previous day, and issued a NOTAM that the runway lights were inoperative.  As the flight proceeded inbound, the captain could not determine whether the lights were on bright or dim and indicated that he keyed the radio to increase the lighting, but was uncertain if the intensity had changed. The captain disengaged the autopilot soon after calling runway in sight when the airplane was about 4.5 miles from the runway 6 threshold after he thought he saw the actual runway.  The captain stated he then looked down to check that the gear was down and, as he looked back out, saw nothing but black in front of the airplane before identifying pavement that was lit up. He stated he then smoothly slid the airplane over a little to line up with the illuminated pavement.  He stated that he thought he had aligned with the runway. The flight crew noticed they were on a taxiway as they were in the flare and touching down on the pavement. During the taxi to the terminal, the flight crew again attempted to illuminate the runway lights via the CTAF with no success, only the taxiway lights changed illumination intensity. Although some cues, such as the taxiway light color and alignment of the PAPI relative to the intended landing surface, were available to the flight crew to distinguish the taxiway from the from nonilluminated runway, sufficient cues also existed, such as a single lighted paved surface similar in width to the runway, to confirm the crew's expectation that the airplane was aligned with the intended landing runway. As a result, once the airplane was aligned with what the flight crew thought was the correct landing surface, they were likely not strongly considering contradictory information.  Once the flight had exited the clouds, Page 2 of 22 DCA18IA081the flight crew had expected to see an illuminated runway.  The taxiway and runway were about 200 feet apart, and only a small correction while in flight would be required to align the aircraft with the taxiway.  The captain indicated that after the runway was in sight, he focused on the runway surface, and not the lighting until the airplane had landed on the taxiway. The NTSB concludes that the cues available to the flight crew to indicate that the airplane was aligned with a taxiway were not sufficient to overcome their belief, as a result of expectation bias, that the illuminated taxiway was the intended landing runway.