[7] Smith made a total of nine appearances for the Mariners in September; he did not allow a run in 8+1⁄3 innings pitched, while recording ten strikeouts and issuing three walks.

[1] For the season, Smith appeared in 70 Mariners games, compiling a 2.31 earned run average (ERA) while recording 13 saves; in 70 innings pitched, he had 92 strikeouts and issued 22 walks.

On December 7, 2015, the Mariners traded Smith and Roenis Elías to the Boston Red Sox for Wade Miley and Jonathan Aro.

[11][12] Smith made three appearances for the Red Sox during the regular season, pitching 2+2⁄3 scoreless innings in May while striking out two and walking one before landing back on the disabled list on May 20 with an elbow injury.

[15] After more than a year recovering from surgery, Smith was sent on rehabilitation assignments in August 2017 with the Double-A Portland Sea Dogs and Triple-A Pawtucket Red Sox.

[17] He appeared in eight regular season games for Boston during September, giving up one run in 6+2⁄3 innings pitched (1.35 ERA) while striking out seven and walking two.

On September 18, 2017, Smith picked up his only save as a member of the Red Sox during an extra inning victory over the Baltimore Orioles.

[20] The injury occurred when Smith, in reaction to allowing a home run to Khris Davis of the Oakland Athletics in the prior day's game, threw his glove in the dugout out of frustration.