| Konspirationstheorie | conspiracy theory |
| Konspirationstheoretiker | conspiracy theorist |
| Konspirationstheoretikerin | conspiracy theorist |
| Informationstheorie | information theory |
| Konspiration | conspiracy |
| Evolutionstheorie | theory of evolution |
| Ionisationsröhre | ionisation tube ionization tube |
| Ionisationstechnik | Ionisation technique ionization technique |
| Staatstheorie | theory of the state |
| Kategorientheorie | category theory |
| Knotentheorie | knot theory |
| Pongidentheorie | Pongidae hypothesis |
| Sprachtheorie | linguistic theory theory of language |
| Stringtheorie | string theory |
| Superstringtheorie | superstring theory |
| Kondensationsflasche | Leyden jar |
| Konditionierer | conditioner |
| Konfektionsgröße | size |
| Konsekration | dedication |
| Konstitutionalistin | constitutionalist |