D.C. United was reduced to 10 men during the third minute of first-half stoppage time when midfielder Russell Canouse earned a red card after a foul on Derrick Jones. It was his second yellow card before the half.
Rios' first career MLS goal, just past the midway point of the second half, came after an Alistair Johnston corner kick that Dave Romney deflected with his head. The ball found Rios, who won a battle with D.C. United defender Joseph Mora to score on a header.
The victory improved Nashville SC's record to 4-5-3 (15 points) and gave them a point in each of their past four home games. Their only defeat in their last six games was Saturday's 2-0 decision at the Columbus Crew, who own the best record in MLS.
Since MLS returned to action in team stadiums following the MLS is Back Tournament, Nashville SC is 4-3-3. They did not participate in the tournament because of a COVID-19 outbreak among the team.
D.C. United (2-6-5, 11 points) is in the midst of a challenging season, but they did enter Wednesday with a point in two of their previous three games.
Goalkeeper Bill Hamid was nearly up to the challenge for D.C. United, making four saves while allowing the one Rios goal. Nashville SC goalkeeper Joe Willis made one save in his fifth shutout of the season and the 23rd in his 10-year career.
Tori Penso was the head referee for the game, making her the first female head referee of an MLS game since 2000.