The 25-year-old has signed a six-and-a-half-year deal to become the first arrival since head coach Abelardo took charge last month.
The deal also includes €2m ($2.3m) in add-ons and Benfica will retain a 20 per cent sell-on clause for the forward.
The fee smashes the club's previous transfer record of €10.5m ($11.6m), which they paid to sign Matias Vargas from Velez Sarsfield last July.
De Tomas, who progressed through the Real Madrid youth system, enjoyed productive loan spells with Real Valladolid and Rayo Vallecano before joining Benfica on a five-year deal in July.
However, he managed only seven appearances in Portugal's top flight and did not score a single goal.
Sevilla were also said to be interested in bringing him back to Spain this month after allowing Munas Dabbur to sign for Hoffenheim.
De Tomas, who scored 14 goals in 33 appearances for Rayo in LaLiga last season, will be tasked with finding the goals to help drag Espanyol out of the relegation zone.
Espanyol have scored just 14 goals in 19 league games this season, two of which came in the 2-2 draw with city rivals Barcelona last Saturday.
Abelardo's side are bottom of LaLiga on 11 points, four adrift of safety, having won just twice all season in the top flight.