Helenius, who arrives at Villa Park from Danish club Aalborg BK, becomes Paul Lambert's fourth new signing of the summer.
The 22-year-old has previously played with fellow new boy Jores Okore in the Denmark international team and Lambert is confident Helenius will fit in perfectly with the young team.
"Nicklas is a young lad but also a full international with a good goalscoring record in Denmark and he'll prove himself in the Premier League," said Lambert.
"The opportunity will be there for him, as it will be for the rest of the lads, and he has all the attributes to go and take it.
"He wants to go and make a name and a career for himself and he's made a good start at Aalborg. It's a case of continuing to work hard and building on that and I believe he can.
"He's a good player and the potential is there for him to grow into the team and grow with the team. He'll definitely be able to give the team a hand."
Helenius, who was named UEFA's Danish Superliga player of the 2012/13 campaign, scored 16 goals in 33 league appearances last season, making seven assists.
Helenius said: "This is a big opportunity for me, a big dream to play in the Premier League with a club with the history of Aston Villa and I'm very excited.
"In Gabby Agbonlahor and Andi Weimann, for example, we have good players who have great speed and energy and a real love for playing for Villa.
"You can see this and you can also see the strength and quality of Christian Benteke. It is really exciting to be a part of this and I'm looking forward so much to pulling on the shirt and getting to know my new team-mates."