Puljujarvi joins a young Oilers core led by star center Connor McDavid.
The 18-year-old forward spent last season with the Finnish Elite League's Oulu Karpat, getting 28 points (13 goals, 15 assists) in 50 games.
He also had nine points (four goals, five assists) in 10 playoff games. Puljujarvi has spent the past two seasons with Karpat, getting 39 points in 71 games.
The Tornio, Finland, native represented his country most recently at the 2016 world junior championship, leading the tournament in scoring with 17 points in seven games, helping Finland capture a gold medal. His 17 points rank second in all-time points for a player under the age of 18 at the tournament, behind Jaromir Jagr and tying Wayne Gretzky and Eric Lindros.