| # | Name | Origin | Meaning | Gender | |
|---|---|---|---|---|---|
| 561 | Isolda | Welsh | Beautiful. Fair. | F | |
| 562 | Isolde | Welsh | Variant of Isolda: Beautiful. Fair. | F | |
| 563 | Ithel | Welsh | Generous lord. | M | |
| 564 | Iustig | Welsh | Legendary son of Caw. | M | |
| 565 | Ivor | Welsh | Variant of Ifor: Lord. Can also be a variant of Ivor. | M | |
| 566 | Iwan | Welsh | Welsh form of John 'God's gift'. | M | |
| 567 | Jennifer | Welsh | Fair one. Variant of Guinevere. In Arthurian Mythology Guinevere was Arthur's queen. | F | |
| 568 | Jesstin | Welsh | Variant of Justin 'Just; upright; righteous.'. | M | |
| 569 | Jestin | Welsh | Variant of Justin 'Just; upright; righteous.'. | M | |
| 570 | Jestina | Welsh | Just; upright. Feminine of Justin. | F | |
| 571 | Jestine | Welsh | Just; upright. Feminine of Justin. | F | |
| 572 | Jeston | Welsh | Variant of Justin 'Just; upright; righteous.'. | M | |
| 573 | Kai | Welsh | Keeper of the keys. Form of Kay. | M | |
| 574 | Kane | Welsh | Beautiful. Popular as a first name in Australia. | M | |
| 575 | Kay | Welsh | Fiery. | M | |
| 576 | Kei | Welsh | Legendary son of Kynyr. | M | |
| 577 | Keith | Welsh | Dwells in the woods. | M | |
| 578 | Kelemon | Welsh | Legendary daughter of Kei. | F | |
| 579 | Kelli | Welsh | From the wood. | M | |
| 580 | Kelyn | Welsh | Legendary son of Caw. | M | |