The former Arsenal and Chelsea defender had become a free agent following the demise of WPS rivals St Louis Athletica last week, having only moved to the club on 5 May from reigning champions Sky Blue FC.
Asante, 25, becomes the third England player to join the Red Stars, alongside national team-mates Karen Carney and Katie Chapman.
"When we looked at Anita, we looked at her experience, versatility and athleticism, and those were the three deciding factors toward bringing her in," said new Red Stars head coach Omid Namazi, who replaced Englishwoman and former Arsenal coach Emma Hayes last week.
"We feel she'll add a lot to the team. We're going to take a look at our needs and see where she can fit in the most. She had been playing centre-back at Saint Louis, and holding midfielder and defender in England. So we're flexible in where we can use her."