| K-Swiss | business, subsidiary company | Westlake Village | 1966 |
| Kahr Arms | business | Pike County | 1995 |
| Kaiser Motors | automobile manufacturer | Ypsilanti | 1945 |
| Kalem Company | film production company | New York City | 1907 |
| Kaman Corporation | public company, aerospace manufacturer | Bloomfield, Connecticut | 1945 |
| Kansas State University | public educational institution of the United States, land-grant university, public university | — | 1863 |
| Katz's Delicatessen | restaurant | — | 1888 |
| Kayak.com | travel website, business | Stamford | 2004 |
| Kaypro | business | San Diego | 1981 |
| Kearney | business, enterprise | Chicago | 1926 |
| Keds | business, subsidiary company | Rockford | 1916 |
| Kel-Tec | business | Cocoa | 1991 |
| Kelvinator | business | — | 1914 |
| Kentucky State University | historically black college or university, public educational institution of the United States, land-grant university, public university | — | 1886 |
| Keurig Dr Pepper | business, enterprise, public company | Burlington | 2018 |
| KeyBank | bank, business, financial institution, enterprise, credit institution, public company | Cleveland | 1825 |
| Keystone Studios | business | Edendale | 1912 |
| KFC | fast food restaurant chain, brand, public company | Louisville | 1930 |
| Kings Island | amusement park | — | 1972 |
| Kirby Company | business | Cleveland | 1914 |
| Klasky Csupo | animation studio | Los Angeles | 1982 |
| Kmart | department store chain, discount store, business, enterprise, public company | Detroit | 1962 |
| Knoll | business, subsidiary company, enterprise, public company | East Greenville | 1938 |
| Koch, Inc. | privately held company, business, enterprise | Wichita | 1940 |
| Kodak | camera manufacturer, holding company, business, enterprise, public company | Rochester, New York City | 1882 |
| Kohlberg Kravis Roberts | investment company, financial services company, asset management company, business, private equity firm | New York City | 1976 |
| Kohler Company | business, enterprise | Kohler | December 6, 1873 |
| Kohn Pedersen Fox | architectural firm | Salmon Tower Building | 1976 |
| Kongregate | video game publisher, website | San Francisco | 2006 |
| Koss Corporation | business, public company | Milwaukee | 1953 |
| Kraft Foods Group | food manufacturer, public company | Northfield | 2012 |
| Kraken | bitcoin exchange, cryptocurrency exchange | San Francisco | 2011 |
| Krispy Kreme | fast food restaurant chain, restaurant chain, public company | Winston-Salem | July 13, 1937 |
| Kroger | supermarket chain, business, gas station chain, enterprise, public company | Cincinnati | 1882 |
| Krystal | fast food restaurant chain | Atlanta | 1932 |