Zaitsev, 25, had 36 points (four goals, 32 assists) in 82 regular-season games and averaged 22:01 per game, second on the Maple Leafs. He was one of six rookies to play at least 75 games for Toronto this season, helping them qualify for the Stanley Cup Playoffs for the first time since 2013.
"We felt that after having him here a year and certainly watching him over the last couple of years, everything we thought about him was what we saw," Maple Leafs general manager Lou Lamoriello said. "The way he plays, the style he plays, the way he takes care of himself, for a defenseman, we felt this was a good decision.
"He's in an elite category as far as how he can be used and how he's trusted, and can really play in all situations if necessary."
During the playoffs, Zaitsev averaged 21:44 of ice time in four games. He missed the first two games of the Eastern Conference First Round series against the Washington Capitals with an upper-body injury. The Maple Leafs lost the series in six games.
Zaitsev played his first season in North America following three seasons with CSKA Moskva (Moscow) of the Kontinental Hockey League.
"It was an experience for him, it was a transition," Lamoriello said. "He came from a larger ice surface and a different style of play and he adjusted tremendously throughout the year. The experience is going to help him, knowing the League better, knowing the players better; he came in completely cold.
"What came quickly was his tenacity, his pushback. His competitive level was higher than maybe I expected in watching him. Unfair to him, because you don't see him a lot prior to coming, you see him on film and see him live two or three times. Each and every night, he competes. That's the thing that really stood out."
Zaitsev could have become a restricted free agent July 1. He signed a one-year entry-level contract on May 2, 2016 that had an NHL salary-cap charge of $1.775 million, according to Cap Friendly.
Smith, 28, had four points (two goals, two assists) in 36 games with the Maple Leafs after being claimed on waivers from the Colorado Avalanche on Oct. 24. Smith could have become an unrestricted free agent on July 1. He played this season on a one-year contract with a salary-cap charge of $675,000, according to Cap Friendly.
"I think we all know the type of character that Ben has and what an asset he is to the organization," Lamoriello said. "It's not a hidden secret that he also serves and is eligible for the [NHL] Expansion Draft and meets the criteria. There was a combination of reasons for this, and he gives depth to the organization."
