The Brazilian goalkeeper has arrived from Famalicao for a club record €12m for a goalkeeper.
Junior said at today's presentation: "I am very happy to be part of a team like Villarreal, of the investment they have made for me and to play in one of the best leagues in the world. They have received me very well.
"I have to continue working every day, as I have done in Portugal and the one who decides who plays is the coach. I don't have to be thinking about that.
"I knew a lot about a club like this that always competes in European leagues. It is a great club and with my performance I hope to help it continue to grow."
He added, "That I am the goalkeeper for whom they have paid the most is not a pressure, it is a motivation. Pressure was what I felt when I was 18 years old at Famalicao and where I had to play and earn a position being so young. "