| # | Name | Origin | Meaning | Gender | |
|---|---|---|---|---|---|
| 601 | Steffon | Welsh | Crown; wreath. Variant of Stephen. | M | |
| 602 | Sugyn | Welsh | Legendary son of Sugynedydd. | M | |
| 603 | Sulyen | Welsh | Legendary son of Iaen. | M | |
| 604 | Syvwkh | Welsh | Legendary son of Cleddyv KyvwIch. | M | |
| 605 | Tad | Welsh | Father. | M | |
| 606 | Tadd | Welsh | Father. | M | |
| 607 | Taffy | Welsh | Diminutive of David: Beloved or friend, from the Hebrew. Sixth century St David (or Dewi) was patron saint of Wales. | M | |
| 608 | Taliesin | Welsh | Handsome; radiant brow. 6th century Welsh bard and author of the Book of Taliesin. | M | |
| 609 | Talyessin | Welsh | Handsome. | M | |
| 610 | Taren | Welsh | Thunder. | M | |
| 611 | Tarran | Welsh | Thunder. | M | |
| 612 | Tarrant | Welsh | Thunder. | M | |
| 613 | Taryn | Welsh | Thunder. | M | |
| 614 | Teithi | Welsh | Legendary son of Gwynnan. | M | |
| 615 | Teregud | Welsh | Legendary son of Iaen. | M | |
| 616 | Tewdwr | Welsh | God's gift. | M | |
| 617 | Timotheus | Welsh | Welsh form of Timothy 'God given'. | M | |
| 618 | Tomos | Welsh | Twin. | M | |
| 619 | Trahaym | Welsh | Incredibly strong. | M | |
| 620 | Trahern | Welsh | Incredibly strong. | M | |