Adam Caplan of SiriusXM NFL Radio first reported the deal. Richardson was ranked No. 41 on NFL.com's Top 101 Free Agents of 2018.
Richardson spent his first four seasons in Seattle after being selected in the second round in 2014. The 6-foot receiver busted out last year, compiling 44 receptions, 703 yards and six TDs.
The Seahawks' loss is a big win for Washington, where Richardson will provide a deep target for Alex Smith opposite Josh Doctson, with Jamison Crowder patrolling the slot. The Redskins' offense lacked a field-stretching option after DeSean Jackson left last season. Now Jay Gruden gets a new, young deep threat to help expand his remade offense.