Mellberg moves to the MLS franchise from IF Brommapojkarna.
"I am excited to join the St. Louis as head coach," said the Swede.
"It is a fantastic opportunity here given the fantastic infrastructure and the direction the organization has taken. I'm really looking forward to building on the foundation already laid here and the massive support from the fans that gives this team the energy to be successful."
St Louis sports director Lutz Pfannenstiel also said: "Olof has an incredible work ethic and a reputation for being disciplined and detail-oriented.
"His leadership qualities as a captain, both at the highest club level and with his national team, will be key to steering our group. Olof's ability to develop young talents could be seen in his Swedish club and we look forward to seeing it here in St. Louis."
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