Darby spent the last three seasons with the Philadelphia Eagles, capturing a Super Bowl with the team while also battling multiple injuries.
The speedy defensive back was a second-round pick of the Buffalo Bills in 2015 and was traded to Philadelphia prior to the 2017 campaign.
Darby flashed Pro Bowl potential in his rookie campaign but has struggled with consistency in coverage since then. Lacking high-level ball skills, he recorded just six interceptions over three years with the Eagles.
The 26-year-old is reportedly joining a Redskins squad in desperate need of an upgrade at cornerback. After trading Quinton Dunbar to the Seattle Seahawks, Washington was left with Kendall Fuller, Fabian Moreau, and Jimmy Moreland atop its depth chart.