Rossi keeps Villarreal on course for top four
The Italy striker netted after he had been brought down in the area with 19 minutes to go at the Madrigal and sent Toni Doblas the wrong way from the spot.
The 24-year-old took his tally for the season to 16 and lifted fourth-placed Villarreal to 57 points, nine ahead of Athletic Bilbao in fifth, with six matches left to play.
Barcelona top the standings with 85, eight ahead of Real Madrid after Saturday's 'clasico' at the Bernabeu.
Villarreal take on Porto in the Europa League semi-finals with the first leg in Portugal on May 5.