Oliveira, who has won 14 caps for his country and played for Portugal at Euro 2012, joins from Benfica where he has struggled to make an impact since first breaking into the first-team as a teenager.
The 23-year-old's arrival solves a problem for Swansea manager Garry Monk who will lose top scorer Wilfried Bony to the African Nations Cup next month.
Ivory Coast striker Bony could miss up to six games in January and February and Bafetimbi Gomis is the only other recognised senior striker in the squad.
"I'm really looking forward to it,'' Oliveira told Swansea's official website.
"I already know quite a bit about Swansea and it was their style of play and the ambience at the club that attracted me
It was an easy decision to make.
"The deal was agreed early so I can spend an extra 10 days in Swansea training with the squad and getting to know my new team-mates.
"That way I hope to be ready to offer my services to the team that much earlier.
"I want to prepare properly for the challenge
It's going to be a big challenge because I have a lot of respect for fellow strikers Wilfried Bony and Bafetimbi Gomis.
"I have to earn the right and respect from the manager and my team-mates on the training ground to play.
"But I'm highly motivated to score goals and help the team.''
Oliveira spent last season on loan at French club Rennes, where he scored eight goals in 30 league appearances, and he has also featured for Portuguese pair Rio Ave and Pacos Ferreira on loan along with Spanish side Deportivo La Coruna.
Source : PA
Source: PA