| 100th Training Division (Leader Development) | airborne infantry division, United States Army infantry division, training division | Fort Knox | 1918 |
| 20th Century Animation | television production company, animation studio, film production company | Century City | February 1994 |
| 20th Century Fox Television | television production company | Los Angeles | 1949 |
| 20th Century Studios | television production company, production company, film production company, film studio | Fox Plaza, Walt Disney Studios | 1985 |
| 21 Club | restaurant | — | 1922 |
| 2K | video game publisher, video game developer | Novato | 2005 |
| 3D Systems | business, public company | Rock Hill | 1986 |
| 3M | business, conglomerate, public company | Maplewood | 1902 |
| 4Kids TV | television production company, television channel | — | September 14, 2002 |
| 88rising | media company, record label | New York City | July 30, 2015 |
| AbbVie | biopharmaceutical company, business, public company | North Chicago | April 10, 2012 |
| ABC Family Worldwide | television production company, business | Burbank | 1990 |
| ABCorp | business | Stamford | 1795 |
| Abercrombie & Fitch | business, clothing store chain, public company | New Albany | 1892 |
| AbioMed | business, public company | Danvers | 1981 |
| About.com | media company, website | — | April 21, 1997 |
| Academy of Art University | private for-profit educational institution, art academy, university | San Francisco | 1929 |
| Academy of Motion Picture Arts and Sciences | film academy | Beverly Hills | May 11, 1927 |
| Academy of Natural Sciences of Drexel University | nonprofit organization, natural history museum, academy of sciences, learned society | Philadelphia | 1812 |
| Ace Hardware | organization, business | Oak Brook | 1924 |
| Activision | video game publisher, video game developer, subsidiary company | Santa Monica | October 1, 1979 |
| Acxiom | business, public company | Wilmington, Conway | 2018 |
| Adaptec | business, enterprise | Sunnyvale | 1981 |
| Adscape | business | San Francisco | 2006 |
| Advance Auto Parts | car parts shop chain, business, public company | Roanoke | 1932 |
| Aeronca | aerospace manufacturer | Cincinnati, Middletown | 1928 |
| Affliction Entertainment | business | Los Angeles | 2008 |
| Aflac | business, public company | Columbus | 1955 |
| Ahwahnee Hotel | lodge | — | 1927 |
| AIGA | regulatory college, nonprofit organization, private press, professional association | New York City | 1914 |
| Air Cargo Carriers | airline | Milwaukee | 1986 |
| Air Force Institute of Technology | staff college, engineering education | — | 1919 |
| Air Wisconsin | airline | Appleton | August 23, 1965 |
| Airbnb | website, business, hotel booking website, enterprise, public company | San Francisco | 2008 |
| Airstream | brand, business | Jackson Center | 1931 |
| Airtable | spreadsheet, website, business | San Francisco | 2012 |
| Airtight Games | video game developer | Redmond | 2004 |
| AirTran Airways | airline | Dallas | 1993 |
| AK Press | book publisher, small press | Chico | 1987 |
| Akamai Technologies | business, enterprise, content delivery network, public company | Cambridge, Massachusetts | 1998 |
| Alabama A & M University | historically black college or university, public educational institution of the United States, school, land-grant university, public university | — | 1875 |
| Alabama State University | public educational institution of the United States, school, public university | — | 1867 |
| Alamo Rent a Car | business | Clayton | 1974 |
| Albert Einstein College of Medicine | university, organization | — | 1955 |
| Albertsons | supermarket chain, business, enterprise, public company | Boise | 1939 |
| Aldi | supermarket chain, brand name | Essen, Mülheim an der Ruhr, Batavia | 1946 |
| Aldus | business | Seattle | February 1, 1984 |
| Alembic Inc | privately held company | Santa Rosa | 1969 |