| Illtyd | Man | Welsh Names |
| Ioan | Man | Welsh Names |
| Iolo | Man | Welsh Names |
| Iorwen | Woman | Welsh Names |
| Iorwerth | Man | Welsh Names |
| Islwyn | Man | Welsh Names |
| Jones | Man | Welsh Names |
| Kiah | Woman | Welsh Names |
| Kynan | Man | Welsh Names |
| Lleu | Man | Welsh Names |
| Lleucu | Woman | Welsh Names |
| Llewellyn | Man | Welsh Names |
| Llinos | Woman | Welsh Names |
| Lloyd | Man | Welsh Names |
| Llyr | Man | Welsh Names |
| Llywelya | Woman | Welsh Names |
| Llywelyn | Man | Welsh Names |
| Lowri | Woman | Welsh Names |
| Lunet | Woman | Welsh Names |
| Mabli | Woman | Welsh Names |