Per ESPN's Adam Schefter, Boston agreed to a one-year contract with the Cardinals.
Boston's reported deal with Arizona comes one month after he initially met with the team. He didn't come away from that visit impressed, telling SiriusXM NFL Radio the Cardinals' offer was "very, very disrespectful."
Last season was the most productive of Boston's four-year NFL career. The 25-year-old tied for fifth among all defensive players with five interceptions as a member of the Los Angeles Chargers and started a career-high 15 games.
Boston spent the first three years of his career with the Carolina Panthers. He was a fourth-round draft pick out of North Carolina in 2014.
After being released by the Panthers last May, Boston signed with the Chargers following a fortuitous encounter at a gas station, via ESPN's Eric D. Williams:
"I crossed two lanes to get to the gas station. I pump my gas. After I pump my gas, I put my wallet in my car. After I put my wallet in my car, there's a guy with three kids and he asked me for a little bit of change, or if I had a dollar or two. I gave him a $20.
"So he tells me thank you so much, and that God put me in his life for a reason. And in my mind I'm saying you're here for a reason, too. So in my mind I'm saying should I ask him, 'L.A. or Buffalo.' And before I could ask him, my man told me, 'Thank you so much. There aren't too many people like you in this world, God's children. My name's Mike Daniels, but people call me L.A.'"
The move paid off as Boston turned into an invaluable piece of the Chargers' secondary. He was able to parlay that success into a new contract with the Cardinals.
Boston wasn't consistently part of the Panthers' starting lineup until 2016. He's recorded 15 passes defensed and seven interceptions in the last two seasons.
The Cardinals will likely use Boston as a backup to Antoine Bethea at free safety. His consistency over the past two seasons makes him an exceptional free-agent signing, especially at this stage of the offseason.