| 100th Training Division (Leader Development) | airborne infantry division, United States Army infantry division, training division | Fort Knox | 1918 |
| 20th Century Studios | television production company, production company, film production company, film studio | Fox Plaza, Walt Disney Studios | 1985 |
| 20th Century Studios Home Entertainment | production company, business | Century City, Los Angeles | 1976 |
| 20th Television Animation | business | Los Angeles | May 19, 1999 |
| 21 Club | restaurant | — | 1922 |
| 2K Sports | video game publisher | Novato | January 25, 2005 |
| 3 Arts Entertainment | production company | Beverly Hills | 1991 |
| 3D Realms | video game developer | Garland | 1994 |
| 3D Systems | business, public company | Rock Hill | 1986 |
| 3dfx Interactive | business, enterprise | San Jose | 1994 |
| 3M | business, conglomerate, public company | Maplewood | 1902 |
| 4Licensing Corporation | television production company, public company | New York City | 1970 |
| 7-Eleven | brand, business, gas station chain, convenience store chain | Dallas, Irving | 1927 |
| 87North Productions | film production company | Santa Monica | April 1, 2019 |
| A+E Global Media | business, enterprise | New York City | 1984 |
| A. O. Smith | business, public company | Milwaukee | 1889 |
| A. T. Cross Company | business, public company | Lincoln | 1846 |
| A.D. Vision | film production company | Houston | 1992 |
| Abbott-Detroit | automobile manufacturer | Detroit | 1909 |
| ABC Signature | television production company | Burbank | March 21, 1985 |
| ABCorp | business | Stamford | 1795 |
| Abercrombie & Fitch | business, clothing store chain, public company | New Albany | 1892 |
| ABKCO Records | record label, business | New York City | 1961 |
| Abrams Books | publishing house, book distributor, business | New York City | 1949 |
| Academia.edu | document repository, social networking service, website, academic publisher | San Francisco | 2008 |
| 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 |
| ACCO Brands | business, public company | Lake Zurich | 1903 |
| Ace Hardware | organization, business | Oak Brook | 1924 |
| Acoustical Society of America | publishing house, scientific society | Melville | 1928 |
| Activision | video game publisher, video game developer, subsidiary company | Santa Monica | October 1, 1979 |
| Actors Studio | drama school | New York City | October 5, 1947 |
| Adelphi University | private not-for-profit educational institution, school, university, organization | — | 1896 |
| AdMob | software company, business | Mountain View | 2006 |
| Adscape | business | San Francisco | 2006 |
| Aeria Games | business | Berlin | 2006 |
| Aeronca | aerospace manufacturer | Cincinnati, Middletown | 1928 |
| Affliction Entertainment | business | Los Angeles | 2008 |
| Aflac | business, public company | Columbus | 1955 |
| Ageia | business | Santa Clara | 2002 |
| Ahwahnee Hotel | lodge | — | 1927 |
| AIGA | regulatory college, nonprofit organization, private press, professional association | New York City | 1914 |
| AIM alliance | joint venture, alliance | — | October 2, 1991 |
| Air Tractor | business | Olney | 1973 |
| Air University | staff college, university | — | 1946 |
| Air Wisconsin | airline | Appleton | August 23, 1965 |
| Airtight Games | video game developer | Redmond | 2004 |
| Akamai Technologies | business, enterprise, content delivery network, public company | Cambridge, Massachusetts | 1998 |