Midfielder Mohammed Abu and defender Axel Sjoberg also will not return.
United is in talks with two players whose contracts are expiring: defender Chris Odoi-Atsem and backup goalkeeper Chris Seitz.
Fisher, 29, spent three seasons in Washington, highlighted by his overlapping runs and attacking prowess during the stretch run of the 2018 campaign. Late that year, however, he suffered a severe knee injury, which, compounded by complications and additional surgeries, sidelined him for the entire 2019 season.
This year, the Jamaican national team member appeared in 15 of 23 matches, starting six.
Abu, 29, joined United early this year on loan from Norwegian club Valerenga. He played well in four starts (six appearances) before suffering a late-season knee injury. His arrangement included a team option to purchase his contract this winter.
Sjoberg, 29, arrived in August in a trade with Columbus and made two appearances (both starts). Fisher and Sjoberg are eligible for MLS free agency.
Fifteen D.C. players were previously under contract for 2021: goalkeeper Bill Hamid; defenders Steven Birnbaum, Frédéric Brillant, Donovan Pines and Jacob Greene; midfielders Paul Arriola, Yamil Asad, Russell Canouse, Edison Flores, Julian Gressel, Moses Nyeman, Kevin Paredes and Griffin Yow; and forwards Erik Sorga and Ola Kamara.
Mora, Moreno, Reyna and Segura combined for 56 appearances and 49 starts this year. Mora and Segura (both from Costa Rica) and Moreno (Venezuela) will return for their fourth MLS seasons. Reyna (Peru) arrived in September in a trade with Vancouver.
Meanwhile, United’s coaching search continues. The team is expected to hire a U.S.-based candidate in the next two weeks, multiple people familiar with the process said.
Candidates include former New York Red Bulls coach Chris Armas; MLS assistants Ezra Hendrickson (Columbus), Pat Noonan (Philadelphia) and Gonzalo Pineda (Seattle); former U.S. women’s coach Jill Ellis; and D.C. interim coach Chad Ashton.
Team officials have spoken to several international candidates as well but seem to be leaning toward a domestic option, two people said.
Read more on soccer: