Phoenix announced Charles' signing Friday, a day after adding Diamond DeShields in a three-team trade with Chicago and New York.
"Tina is an elite talent in our league and adding a player of her caliber is another example of our commitment to our fans and players to pursue a championship every year," Mercury general manager Jim Pitman said in a statement. "Tina has made it clear that she wants to win and wants to do so in Phoenix."
Charles led the WNBA in scoring for the second time in her career in 2021, averaging 23.5 points per game while shooting 36% from the 3-point arc.
An 11-year WNBA veteran, Charles was the league MVP in 2012 and is a nine-time All-WNBA selection and eight-time All-Star. She has led the league in rebounding four times and is a three-time Olympic gold medalist.
Charles was the first overall pick in the 2010 WNBA draft by the Connecticut Sun after winning two national championships at UConn and was the WNBA Rookie of the Year. She played four seasons with the Sun, five with the New York Liberty and last season with the Washington Mystics after receiving a medical opt-out in 2020.