| # | Name | Origin | Meaning | Gender | |
|---|---|---|---|---|---|
| 661 | Mallolwch | Welsh | Legendary king of Ireland. | M | |
| 662 | Manawydan | Welsh | Legendary son of LIyr. | M | |
| 663 | March | Welsh | Legendary son of Meirchywn. | M | |
| 664 | Maredud | Welsh | Magnificent. | F | |
| 665 | Marged | Welsh | Pearl. | F | |
| 666 | Margred | Welsh | Pearl. | F | |
| 667 | Mari | Welsh | Variant of Mary, meaning bitter. Favored prefix for blended names like Maribel. | F | |
| 668 | Marvin | Welsh | Variant of Mervyn: Form of Merlin: From the sea fortress. | M | |
| 669 | Marvyn | Welsh | Variant of Mervyn: Form of Merlin: From the sea fortress. | M | |
| 670 | Mathias | Welsh | Gift of God. | M | |
| 671 | Mawrth | Welsh | Welsh form of Mars, god of war. | M | |
| 672 | Maygan | Welsh | Variant of Margaret - pearl - based on the abbreviation Meg. | F | |
| 673 | Meagan | Welsh | Variant of Margaret - pearl - based on the abbreviation Meg. | F | |
| 674 | Meaghan | Welsh | Variant of Margaret - pearl - based on the abbreviation Meg. | F | |
| 675 | Medyr | Welsh | Legendary son of Medyredydd. | M | |
| 676 | Meeghan | Welsh | Variant of Margaret - pearl - based on the abbreviation Meg. | F | |
| 677 | Meggan | Welsh | Variant of Margaret - pearl - based on the abbreviation Meg. | F | |
| 678 | Meghan | Welsh | Pearl. | F | |
| 679 | Meilyg | Welsh | Legendary son of Caw. | M | |
| 680 | Meirion | Welsh | Name derived from the old county of Merionethshire. | F | |