After selling teenage sensation Joao Felix to Atletico Madrid in a €126m ($142m) deal midweek, Primeira Liga holders Benfica turned to De Tomas on a five-year contract with a €100m ($112m) release clause.
De Tomas emerged from Madrid's youth team but never made a senior appearance – the 24-year-old enjoying loan spells with Cordoba, Real Valladolid and Rayo Vallecano in Spain.
"I'm very happy to be here. After all, it was a goal I had, to make a leap in my career and this was the best leap I could make. I am at the big club, for me it is the best club in Portugal and I'm really looking forward to getting started," De Tomas told BTV.
"I want to do my job and score goals, which is what I do best. I've been a goalscorer since I was a kid and I hope I can score a lot of goals with this jersey and that the team can win titles."
De Tomas spent the past two seasons on loan at Vallecano, where he scored a combined 38 league goals and 14 in LaLiga last term following their promotion.
"May we win the Championship again and do a good job in the Champions League," De Tomas added. "I feel like it's going to be a very positive year. I'm aware that it's hard to win a competitive league, and also because of that there is great merit in Benfica's work last season."