The 21-year-old has represented Benfica since 2015, although all 50 of his league appearances have come for Benfica B.
The forward, who has played for Portugal at both Under-19 and Under-20 level, has revealed that "the size of the club" made it easy for him to choose to join Rangers ahead of a number of interested parties.
"What attracted me was the history of the club, the size of the club, and mainly, the supporters – the amazing fans the club has," Dalcio told Rangers TV.
"The gaffer was a real influence on me too. I know he is really competitive and ambitious, and that was the reasons I wanted to join him. The manager has said he is here to help me and to improve me. On the club, we need to put it on top where it belongs."