Smith was traded to the Mariners on Monday, Jon Heyman of CBS Sports reports.
Smith was part of a deal that also sent Roenis Elias to Boston in exchange for Wade Miley and Jonathan Aro. The 26-year-old put up impressive numbers in his first full season last year, working to a 2.31 ERA with a strong 92:22 K:BB ratio in 70 innings and figures to see high leverage opportunities in the back end of the Boston bullpen.