The veteran was hurt during an informal practice period. Coach John Harbaugh said Friday that Foxworth tore his ACL without being touched.
Foxworth had four interceptions last season.
His injury is another setback for Baltimore's defensive backfield. Fabian Washington and Lardarius Webb are dealing with injuries, and safety Ed Reed, who had hip surgery in the offseason, will start the season on the physically unable to perform list.
Information from The Associated Press was used in this report.