Smith was entering the final season of a two-year contract; the new deal starts with the 2013-14 season and keeps him with the club through 2016-17.
"It's very exciting," Smith told reporters by conference call. "I got a call from my agent two weeks ago saying Ottawa was interested in doing a long-term deal. It's pretty exciting and very flattering that they want me around for another four years and that [they think] I fit well in the overall scheme of things. I couldn't be more happy to get the deal done. It's a great ending to the summer."
A 2008 third-round pick of the Senators, Smith had 14 goals, 12 assists and 98 penalty minutes in 81 games last season, his first full NHL campaign. He also had one assist in seven games in the 2012 Stanley Cup Playoffs.
In 152 NHL games spread across the past four seasons, Smith has 20 goals and 18 assists.
"The one thing we know about the NHL [is that] you have to be a competitive person and you have to be strong and you have to be willing to mix it up and protect your teammates once in a while," Senators general manager Bryan Murray said. "Zack certainly has shown an ability to do that and a willingness to do that, and when you get that physical presence, along with some ability to get points and to be a productive player ... those are the guys who are harder to find than maybe any other type.
"It’s hard to find good young NHL players and when you get them, you want to make sure you have them for term if at all possible. It allows us to have another good young player to build around."
Murray believes the best is still to come from Smith.
"We think he's an improving player," Murray said. "We know that last year, being his first real NHL season, that the inconsistency [he showed] at times was a matter of youth. We think that is the one area he will get better at. Likewise, we think he will get stronger and more NHL ready.
"There's no question he has the ability to be a strong checker. He's got size, he's got defensive awareness, and we think he competes real hard with and without the puck. We think he can match up against good players on another team and become a shut-down type of player. But he's also shown the ability to score points."