| Siarl | Welsh | Welsh cognate of Charles (full-grow.. |  |  |
| Sieffre | Welsh | Welsh cognate of Geoffrey. See GEO.. | | |
| Siencyn | Welsh | Welsh form of Jenkin (God is gracio.. | | |
| Sion | Welsh | Popular Welsh cognate of John (God .. | | |
| Sioned | Welsh | Welsh form of Janet, which is a dim.. |  | |
| Sior | Welsh | Welsh form of George (earthworker, .. | | |
| Steffan | Welsh | Welsh cognate of Stephen (a crown, .. | | |
| Stephen | Welsh | A borrowing from the English, Steph.. |  | |
| Stuart | Welsh | A borrowing from the English and th.. | | |
| Sulwen | Welsh | A name that signifies "fair as the .. | | |
| Talfryn | Welsh | Compound name composed of the Welsh.. | | |
| Taliesin | Welsh | Compound name composed of the Welsh.. | | |
| Tarrant | Welsh | Meaning "thunder." | | |
| Tegwen | Welsh | Compound name composed of the eleme.. | | |
| Teleri | Welsh | Elaboration of Eleri, an ancient na.. | | |
| Tewdwr | Welsh | Welsh form of Theodore (God's gift).. | | |
| Theodore | Welsh | A borrowing from the English, Theod.. | | |
| Thomas | Welsh | From the Ecclesiastic Greek Thomas,.. |  | |
| Timothy | Welsh | A borrowing from the English, Timot.. | | |
| Tirion | Welsh | Directly derived from tirion (gentl.. | | |
| Tomos | Welsh | Welsh form of Thomas (a twin). See .. | | |
| Trahaearn | Welsh | Common Welsh name formed by add.. | | |
| Trahern | Welsh | This name has the meaning "strong a.. | | |
| Traherne | Welsh | Anglicized form of the Welsh Trahae.. | | |
| Trefor | Welsh | Transferred use of the surname orig.. | | |
| Tristan | Welsh | From the Old French Tristran, which.. | | |
| Tudor | Welsh | Anglicized form of Tudur (ruler of .. | | |
| Tudur | Welsh | Derived from the Old Celtic Teutori.. | | |
| Urien | Welsh | A common name of uncertain derivati.. | | |
| Vaughn | Welsh | Transferred use of the surname orig.. | | |
| Victor | Welsh | A borrowing from the English, Victo.. | | |
| Walter | Welsh | A borrowing from the English, Walte.. | | |
| Wendy | Welsh | A modern pet form of Gwendolen (w.. | | |
| William | Welsh | A borrowing from the English, Willi.. | | |
| Winifred | Welsh | Anglicized form of the Welsh Gwenfr.. | | |
