The team is in rebuilding mode after finishing the 2013 season with a league-worst 3-24-7 record. De Rosario made US$645,333 this season, the highest salary on the team's payroll.
"So humbled by all the support & appreciate your words of encouragement," De Rosario posted on his Twitter account after United's decision was made official. "A new chapter is soon to begin. Stay tuned & have a Happy Halloween."
De Rosario is now eligible for the MLS re-entry draft, where teams can select players who are out of contract or have had their options declined by their current clubs.
"We want to thank Dwayne for his contributions to our team over the past three seasons," said D.C. United general manager Dave Kasper. "He was instrumental in helping the club reach the 2013 Eastern Conference Championship Series, as well as winning this year's U.S. Open Cup.
"This was a difficult decision to make, but it was one that we felt was best for our team to move forward into next season and beyond. We wish Dwayne all the best with his future."
De Rosario started his MLS career in 2001 with the San Jose Earthquakes, where he helped the team to two MLS championships over five seasons.
De Rosario, along with the rest of his Earthquakes teammates, moved to the expansion Houston Dynamo in 2006, where he won two more MLS titles.
Houston traded De Rosario to his hometown team, Toronto FC, on Dec. 12, 2008. He was named captain of the team for the 2010 season after the retirement of Jim Brennan.
His best MLS season came in 2011, when he played for three different clubs. He started the year in Toronto, was dealt to the New York Red Bulls and then traded again to United in mid-season.
In his first 18 games with United, De Rosario recorded 13 goals and seven assists.
De Rosario finished the 2011 season with 16 goals and 12 assists in 33 games with Toronto, New York and D.C. He won the league's most valuable player award as well as the Golden Boot as top goal-scorer.
He was named United's captain in 2012, scoring seven goals and adding 12 assists that season.
While his numbers dipped to three goals and two assists this past season, he still helped United win its third Lamar Hunt US Open Cup, scoring five goals in the process.
De Rosario is currently sixth on the all-time MLS scoring list with 103 goals.
Internationally, De Rosario has 20 goals in 74 appearances for Canada, and has served as the team's captain.