Griffey joins an ownership group that has undergone significant changes over the past two years. Among other people who form part of the group are Seattle Seahawks quarterback Russell Wilson, his wife and singer Ciara and rapper Macklemore.
The Griffey family's ownership stake was finalized earlier this year, before the coronavirus pandemic.
"Ken Griffey Jr. is a name that requires no introduction, whether in Seattle or across the sports world, and we couldn't be more proud to welcome Ken and his family into our club," Sounders FC majority owner Adrian Hanauer said in a statement Tuesday.
"As a lifelong resident of this great city, bringing someone like Ken into Sounders FC holds a special significance to me, as he is truly a part of Seattle sports' heart and soul. We value the contributions that he will add as a partner in our club and welcome him and his family with open arms."
Griffey's MLB career spanned 22 years, with 13 of those seasons coming with the Mariners. The 13-time All-Star selection won 10 Gold Glove Awards and was the 1997 American League MVP.
Griffey finished his career with 2,781 hits, 1,836 RBIs and 630 home runs -- the seventh-most in major league history. He was inducted into the Hall of Fame in 2016.
"This is a meaningful day for our entire family," Griffey said in a statement. "My wife Melissa and I said that we wanted to be a part of something special, not only for ourselves, but for our kids to be proud of, and this is it.
"We know how much this club means to Sounders fans, and for us, this comes back to the city where it all started for our family. We are joining a first-class organization, and we are so proud to give back to the city we love."