After selling teenage sensation Joao Felix to Atletico Madrid in a €126m (£113m/$142m) deal, Primeira Liga holders Benfica turned to De Tomas on a five-year contract with a €100m (£90m/$113m) release clause.
"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 La Liga 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."
Real are looking to trim down their squad after a series of big money moves in the transfer market which saw Luka Jovic, Rodrygo and Eden Hazard arrive at Santiago Bernabeu in the space of seven days recently, plus the earlier additions of Eder Militao and Ferland Mendy.
This has left manager Zinedine Zidane keen to streamline his bulky squad as the club plans to step up their interest in unsettled Manchester United midfielder Paul Pogba.
Real have already spent big this summer in a massive rebuilding programme and will now look to recoup money through sales ahead of focusing their attention on the French World Cup star, who is their top summer target.