Terms of the signing weren't immediately released.
Ian Rapoport of NFL Network reported the Panthers "kept the discussions to football" and did not talk about a potential protest during the national anthem.
Reid spent the first five seasons of his career with the San Francisco 49ers, who selected him in the first round of the 2013 draft. He's tallied 318 combined tackles, 34 passes defended and 10 interceptions across 70 games since arriving to the NFL from LSU. He earned a Pro Bowl selection his rookie year.
Matt Barrows of The Athletic reported the 49ers approached Reid about a reunion on a one-year contract before he signed with the Panthers.
The 26-year-old Louisiana native is coming off a 2017 campaign where he racked up 67 tackles and two picks in 13 appearances for the Niners. Pro Football Focus rated him as the league's 31st-best safety.
Reid entered the offseason facing uncertainty about his future despite the consistent production. He said in late December his decision to kneel alongside former teammate Colin Kaepernick, who went unsigned in 2017, brought the prospect of being left on the free-agent market into the equation.
"I wouldn't use the word concerned," Reid told reporters. "I would say I understand that's a possibility. And I'm completely fine with it. The things that I've done, I stand by, and I've done that for my own personal beliefs. Like I said, I'm fine with whatever outcome happens because of that."
He filed a collusion grievance against NFL in early May after going unsigned through the first six weeks of free agency. Per Rapoport, that grievance "remains unchanged" after signing with the Panthers.
Niners head coach Kyle Shanahan had confirmed in November he wanted Reid to remain part of the long-term plan on defense but noted there's always a "business aspect" of every decision.
"We'll see how this offseason goes, but I know based off the player and person he's definitely a guy I would like back here," he said.
In the end, Shanahan didn't get his wish as Reid leaves to join the Panthers. Not only is it an upgrade for his new team during the season's first month, but the Niners continue to struggle fixing their defense, which ranks 22nd in yards allowed per game after finishing 24th in that category in 2017.
Although Reid shifted to linebacker at times for San Francisco last year, he should return to his typical strong safety spot after signing with Carolina. Look for him to slot in alongside James Bradberry, Captain Munnerlyn and Mike Adams in the secondary after Da'Norris Searcy was placed on IR last week.