The French center back spent two seasons in New York, appearing in more than 50 regular season games and starting in all four of the team’s playoff games during his tenure.
Brillant, 32, signed a two-year deal with D.C. after NYCFC declined his option.
"One of our top priorities this offseason was acquiring an experienced central defender and we are very pleased to complete this transfer," said Dave Kasper, United’s general manager, in their team statement. "Frederic is a smart, athletic defender who is a very good passer. His size, speed, and presence will strengthen our backline next season and we are excited to welcome him to D.C."