| # | Name | Origin | Meaning | Gender | |
|---|---|---|---|---|---|
| 481 | Mathias | Welsh | Gift of God. | M | |
| 482 | Mawrth | Welsh | Welsh form of Mars, god of war. | M | |
| 483 | Medyr | Welsh | Legendary son of Medyredydd. | M | |
| 484 | Meilyg | Welsh | Legendary son of Caw. | M | |
| 485 | Meirion | Welsh | Name derived from the old county of Merionethshire. | M | |
| 486 | Menw | Welsh | Legendary son of Teinkaedd. | M | |
| 487 | Mercher | Welsh | Welsh form of Mercury, messenger of the gods. | M | |
| 488 | Meredith | Welsh | Guardian of the sea. Great chief. This name can be given to children of either gender, but is more common among girls. | M | |
| 489 | Meredydd | Welsh | Guardian of the sea. | M | |
| 490 | Meridith | Welsh | Guardian of the sea. | M | |
| 491 | Merlin | Welsh | Hawk. From the sea fortress. In Arthurian legend, Merlin Ambrosius was adviser/mentor to King Arthur. | M | |
| 492 | Merlyn | Welsh | Sea fortress. In Arthurian mythology the wizard Merlin was King Arthur's mentor. | M | |
| 493 | Merrick | Welsh | Dark-skinned; A Moor. Form of Maurice. | M | |
| 494 | Merrion | Welsh | Variant of Meirion: Name derived from the old county of Merionethshire. | M | |
| 495 | Merry | Welsh | Diminutive of Meredith: Guardian of the sea. Great chief. This name can be given to children of either gender, but is more common among girls. | M | |
| 496 | Merv | Welsh | Diminutive of Mervyn: Form of Merlin: From the sea fortress. | M | |
| 497 | Mervin | Welsh | Variant of Mervyn: Form of Merlin: From the sea fortress. | M | |
| 498 | Mervyn | Welsh | Form of Merlin: From the sea fortress. | M | |
| 499 | Merwyn | Welsh | Form of Merlin. | M | |
| 500 | Meurig | Welsh | Dark-skinned; A Moor. Form of Morris. | M | |