Playing under the franchise tag, Joyner will earn nearly $11.3 million and is scheduled to become a free agent after the season.
A fifth-year pro, Joyner played his first three seasons at cornerback before defensive coordinator Wade Phillips moved him to safety.
Joyner, 27, thrived at the position and finished as the third-highest-graded safety by Pro Football Focus. Joyner intercepted three passes, returning one for a touchdown, had nine pass deflections and a forced fumble.
Following an 11-5 season and their first playoff appearance since 2004, the Rams opted to place the franchise tag on Joyner rather than receiver Sammy Watkins, who went on to sign a free-agent deal with the Kansas City Chiefs.
Earlier this offseason, Joyner said he was in a "good place" with the tag, but said that the goal was to "get a long-term deal done."
The Rams have about $2.29 million remaining in salary-cap space, according to ESPN Stats & Information, and remain in the process of working on a long-term extension for reigning NFL Defensive Player of the Year Aaron Donald, who is scheduled to earn $6.9 million in the final season of his rookie contract.
Of the five players placed under the franchise tag this offseason, including Joyner, Pittsburgh Steelers running back Le'Veon Bell, Dallas Cowboys defensive end DeMarcus Lawrence, Detroit Lions defensive end Ziggy Ansah and Cleveland Browns receiver Jarvis Landry, Landry is the only player to sign a long-term deal.
The Rams could place a second franchise tag on Joyner following the 2018 season, a move they made in 2016 and 2017 with cornerback Trumaine Johnson, who signed a long-term contract this offseason with the New York Jets.
The Rams open training camp on July 26.