Smith's deal is for two years and worth $3.425 million per year, while Krug's is one year and worth $3.4 million.
Smith, 23, has scored 12 goals and tallied 23 assists in 63 games this season, and is tied for third on the team in points. He is in his second year with the Bruins after two years with the Dallas Stars.
Krug, 23, has 11 goals and 20 assists in 59 games this season. He made his NHL debut with Boston in 2012.
The Bruins (31-22-10) host the Philadelphia Flyers on Saturday.
- Molly Geary