| Johana | | - | Czech | |
| Jolana | | - | Czech, Slovak | |
| JonÁŠ | | - | Czech, Slovak | |
| Josef | | YO-zef (German) | German, Swedish, Norwegian, Danish, Czech | |
| Josefa | | ho-SE-fah (Spanish) | Spanish, Portuguese, Czech | |
| Judita | | - | Lithuanian, Czech, Slovak | |
| Julie | | zhuy-LEE (French), YOO-lee-e (Danish), YUW-lye (Czech), JOO-lee (English) | French, Danish, Norwegian, Czech, English, German, Dutch, Spanish, Portuguese | |
| JustÝNa | | - | Czech | |
| Justina | | jus-TEE-nə (English) | English, Slovene, Czech, Lithuanian, Late Roman | |
| KÁJa | | - | Czech | |
| KajetÁN | | - | Czech, Slovak, Hungarian | |
| Kamil (2) | | KAH-meel (Polish) | Czech, Slovak, Polish | |
| Kamila | | kah-MI-lah (Czech, Slovak), kah-MEE-lah (Polish) | Czech, Slovak, Polish | |
| Karel | | KAH-rəl (Dutch), KAH-rel (Czech) | Dutch, Czech, Slovene | |
| KarolÍNa | | - | Czech | |