Osasuna secured their first away win of the season thanks to goals either side of half-time from Roland Lamah and Javad Nekounam.
After a poor opening Lamah raced on to an excellent through ball from Raul Garcia and finished calmly to give the visitors the lead two minutes before the break.
The Belgian was also involved in Osasuna's second four minutes into the second period as he laid the ball back for Nekounam to strike the ball in off the post from 25 yards.
Yet, just as the visitors were looking comfortable, a mistake from goalkeeper Andres Fernandez allowed Alvaro Vasquez to pull one back for Espanyol.
The hosts dominated possession for the last half hour but were unable to carve out a clear opportunity, and despite having Eneko Satrustegui sent-off with 15 minutes remaining, Osasuna held on to move into the top-half.
Espanyol have struggled for goals all season, with Alvaro's profligacy being one of the main issues, and the Spanish Under-21 international was again off target with the best chance of the opening quarter as Joan Verdu's perfectly directed cross found him completely unmarked inside the area, but his flicked header floated just wide.
Jose Luis Mendilibar had dropped Nino in favour of the pace and strength of Lamah and Ibrahima Balde in attack and that decision paid dividends minutes before the interval as Lamah scored his first goal for the club.
Raul Garcia has been involved in over 50% of Osasuna's goals this season and the on-loan Atletico Madrid midfielder provided another assist as he intercepted a ball in midfield and instantly released Lamah clean through on Cristian Alvarez. The Argentinian goalkeeper could have done more to narrow the angle but opted to stay on his line and Lamah easily stroked the ball into the corner.
Osasuna's second was more spectacular as Nekounam rifled in a trademark long-range effort high into the top corner just four minutes after the break.
Within five minutes though the deficit had been halved as Fernandez came to punch away Verdu's cross but was beaten to the ball by Alvaro and his header dropped into the empty net.
An injury to Sergio Garcia had allowed Vladimir Weiss a rare start and the on-loan Manchester City winger came to life as the game progressed.
Fernandez redeemed himself slightly as he tipped over a rising drive from the Slovakian before Satrustegui was sent-off for the second time in only his second game for Osasuna when he was shown a second yellow card for impeding another Weiss run.
With the man advantage Espanyol pressed hard for an equaliser, but the lack of craft that has characterised the season was in evidence as they failed to break the visitors down.
Victory means Osasuna leapfrog their opponents and move up into eighth.