Alonso, 23, defected - along with several teammates - from the Cuban national team during the 2007 Gold Cup and found refuge in Seattle. Along with his friends and family in Cuba he left behind his club, FC Pinar del Rio, for which he had played three seasons.
Seattle purchased his rights from Charleston. The fee wasn't disclosed, though such deals normally involve relatively small sums ($50,000 or less) and a commitment for the teams to play each other once or twice.
Alonso started 31 games for the Battery and in addition to his four regular-season goals tallied two more in the U.S. Open Cup during Charleston's run to the final, in which it lost to D.C. United.
He played 17 times for the Cuban national team and also captained his country's U-23 team during the Concacaf Olympic qualifiers last year.
"It's great to get a player like Alonso," said Sounders FC coach Sigi Schmid in a release announcing the deal. "As a technical central midfielder, he's very good with the ball, a clever passer and he's coming off a very successful USL season."
The Sounders began training last week and will head to Southern California in early February to play its first preseason games against the Galaxy (Feb. 9) at Home Depot Center and Shandong Luneng of China the following day at the Sounders' training camp in Oxnard.
Seattle will also play the Ventura County Fusion (PDL) on Feb. 11, and the San Jose Earthquakes in San Luis Obispo Feb. 13 before heading back north.