- THOMAS, WILLIAM (1734 - 1799), cleric and antiquary
- THOMAS, WILLIAM (1613 - 1689), bishop
- THOMAS, WILLIAM (1723 - 1811), Calvinistic Methodist exhorter
- THOMAS, WILLIAM (1790 - 1861), poet
- THOMAS, WILLIAM (KEINION) (1856 - 1932), Congregational minister, and publicist
- THOMAS, WILLIAM (1727 - 1795), schoolmaster and diarist
- THOMAS, WILLIAM (fl. 1843), colliery owner - see LEWIS, Sir WILLIAM THOMAS
- THOMAS, WILLIAM (1891 - 1958), under-secretary, Ministry of Housing and Local Government
- THOMAS, WILLIAM DAVIES (1889 - 1954), Professor of English
- THOMAS, Sir WILLIAM JAMES (1867 - 1945), BARONET, coalowner, philanthropist
- THOMAS, WILLIAM JENKYN (1870 - 1959), schoolmaster and author
- THOMAS, WILLIAM MEREDITH (1819 - 1877), sculptor - see THOMAS, JOHN EVAN
- THOMAS, WILLIAM PHILLIP (Gwilym Rhondda; 1861 - 1954), colliery official
- THOMAS, WILLIAM THELWALL (1865 - 1927), surgeon
- THOMAS, WILLIAM THEOPHILUS (Gwilym Gwenffrwd; 1824 - 1899), Congregational minister and poet
- THOMAS, ZACHARIAS (1727 - 1816), Baptist minister
- THOMAS, ZACHARIAS - see THOMAS, ZECHARIAS
- THOMPSON, DAVID (1770 - 1857), colonial surveyor and explorer in British North America
- TIBBOT, JOHN (c. 1757 - 1820), clock-maker - see TIBBOTT
- TIBBOTT family
- TILLEY, ALBERT (1896 - 1957), mace-bearer at Brecon cathedral and local historian
- TOM FAIRPLAY - see Williams, Thomas
- TOM FARMER - see THOMAS, THOMAS
- TOM NEFYN - see WILLIAMS, THOMAS
- TOM SPAIN - see JONES, THOMAS
- TOMAS ap IEUAN ap RHYS (c. 1510 - 1617), writer of cwndidau (short religious songs or carols)
- TOMAS ap LLYWELYN ap DAFYDD ap HYWEL - see LLYWELYN, TOMAS
- TOMKINS family, musicians
- TOMKINS, GILLES (d. 1668), organist - see TOMKINS family
- TOMKINS, JOHN (c. 1586 - 1638), college organist - see TOMKINS family
- TOMKINS, ROBERT (fl. 1633), musician - see TOMKINS family
- TOMKINS, THOMAS (c. 1545 - c. 1626/7), organist - see TOMKINS family
- TOMKINS, THOMAS (1572 - 1656), composer - see TOMKINS family
- TOMLEY, JOHN EDWARD (1874 - 1951), solicitor
- TOMOS ap TITUS - see WADE-EVANS, ARTHUR WADE
- TOMOS GLYN COTHI - see EVANS, THOMAS
- TONYPANDY, Viscount - see THOMAS, THOMAS GEORGE
- TOUT, THOMAS FREDERICK (1855 - 1929), historian
- TOY, HUMFREY (d. 1575), merchant
- TRACY, CHARLES HANBURY (1777 - 1858), industrialist - see HANBURYfamily, industrialists
- TRAHAEARN ap CARADOG (d. 1081), king of Gwynedd
- TRAHAEARN BRYDYDD MAWR (fl. first half of the 14th century), poet
- TRAHERNE, JOHN MONTGOMERY (1788 - 1860), antiquary
- TRAINER, JAMES (1863 - 1915?), Association footballer (the 'prince of goalkeepers')
- TREBOR ALED - see JONES, ROBERT
- TREBOR MAI - see WILLIAMS, ROBERT
- TREDEGAR, 2nd Viscount - see MORGAN, EVAN FREDERIC
- TREE, RONALD JAMES (1914 - 1970), priest and schoolmaster
- TREFGARNE of Cleddau, 1st Baron - see TREFGARNE, GEORGE MORGAN
- TREFGARNE, GEORGE MORGAN (1st BARON TREFGARNE of Cleddau), (1894 - 1960), barrister-at-law and politician