Washington replaces Mike Gallego, who was relieved of his duties following a seven-year run as third base and infield coach with the team.
After serving as a coach with the A's from 1996-2006, Washington returned to the organization in May as a fielding instructor. While with Oakland previously, he was credited with developing the fielding talents of six-time Gold Glove winner Eric Chavez and former MVP Miguel Tejada.
Washington, 63, was also the manager of the Texas Rangers from 2007-2014.
Oakland currently sits in last place in the American League West, trailing the Houston Astros by 15 games.
- Kayla Lombardo