The 24-year old spent two years with Sporting, scoring 38 goals in 50 appearances.
"Some people may see my transfer as a step back," he said. "But the Premier League is bigger than the Portuguese league. Life is good in Portugal, but the football has to be on a certain level too.
"Sporting ended this season as seventh in the league; the club has not won prizes for many years. Sporting remains a big club of course, but especially because of its history.
"Norwich is a small club compared to Sporting, but also a friendly club with a beautiful stadium and good fans. That suits me.
"The manager said I will be his number one striker. I know have to battle for it, but it's always good to get the confidence."