The news was first reported by Fox Sports and confirmed by ESPN sources.
The Rams and Johnson can continue to negotiate a long-term contract until July 15. If no deal is reached, Johnson will play a second straight season under the franchise tender. Had Johnson not signed his tender, he could have signed an offer sheet with another team, but that club would have had to surrender two first-round draft picks if the Rams didn't match the offer.
"I feel blessed, honored and really grateful to the organization for placing the tag on me for the second straight year," Johnson told the team website, thanking owner Stan Kroenke, general manager Les Snead and new coach Sean McVay. "... I'm thankful. This is my third contract, and I'm back with my brothers, my teammates, again. I'm glad that I get to come back and play with these guys another season, and I'm excited to be back."
The Rams slapped the franchise tag on Johnson last year, paying him $13.952 million while letting their former No. 1 corner, Janoris Jenkins, leave to the Giants with a five-year, $62.5 million contract. Franchising Johnson again illustrated the Rams' desperate need to preserve depth in their secondary, especially with their starting strong safety, T.J. McDonald, set to become an unrestricted free agent.
The tag saps about half of the Rams' salary cap space, leaving them with roughly $20 million in an offseason when they don't have a first-round pick and have several other needs -- at wide receiver, offensive line, linebacker and safety.
But it would also give Johnson a one-year tryout to see if he is a fit under new defensive coordinator Wade Phillips, who will convert the Rams from a 4-3 to a 3-4.
"Wade values corners," Snead said at the combine last week, "so I think what we're going to do with Trumaine is, because everyone is new, we need to work together, live together, see if we all fit. Does Tru fit Wade? Does Wade fit Tru? Because it's obvious by the tag number and what corners get paid, it's a heavy investment and you want to be right - especially when you go long term."
Johnson, a third-round pick out of Montana in 2012, is the first cornerback to be tagged in back-to-back years since Charles Woodson in 2004 and '05.
The 27-year-old intercepted 15 passes from 2012 to '15, tied for fourth in the NFL during that time. His interceptions dropped from seven in 2015 to one in 2016, his first year as the primary corner. But Johnson still graded out 26th among 112 qualified corners by Pro Football Focus despite missing a couple of games with an ankle injury.
Johnson, listed at 6-foot-2 and 208 pounds, is the Rams' best cover corner by a significant margin and is also the only one with real size and length. The next four on their depth chart -- E.J. Gaines, Lamarcus Joyner, Troy Hill and Blake Countess -- are 5-foot-10 or shorter.
Asked about possibly going two straight offseasons without getting a long-term deal done with Johnson, Snead referenced his high interception total in 2015, which drove up his price, and the inclination to give him a test run under Phillips in 2016.
The Rams are not ruling out an extension for Johnson, but probably won't engage in those talks until after OTAs.
"We want to proceed with caution before we go long term," Snead said. "We communicated this with Tru, and I think everybody's on the same page. 'Let's work through.' And I'm not saying this year. We still have time to get things done, but we need to hit the grass and make sure that we're syncing up."
ESPN's Adam Schefter contributed to this report.