The 28-year-old is headed to the nation's capital after spending the past two seasons with La Liga side Almeria.
The 28-year-old Spaniard joins United after spending the past two seasons with Spanish top-flight side Almeria, making 36 appearances and scoring six goals. Fernandez previously played for Racing Santander in Spain.
"We are pleased to be adding a left back with solid La Liga experience who is in the prime of his career," D.C. United general manager Dave Kasper said in a news release. "Christian is very good at getting forward to join the attack and providing quality service. This move helps solidify our back line moving into the new season."
Kasper first teased United's pursuit of a left back at last month's MLS SuperDraft. Fernandez figures to step into an entirely rebuilt D.C. back line, joining fellow newcomers Jeff Parke, Bobby Boswell and Sean Franklin.
United has added Steve Birnbaum and Nana Attakora as defensive depth as well during an offseason also featuring the acquisitions of strikers Eddie Johnson and Fabian Espindola along with midfielders Davy Arnaud and Christiano Francois.