- SIÔN ap y BEDO ap DAFYDD ap HYWEL ap TUDUR - see SION CERI
- SION BRWYNOG (d. 1567?), poet
- SION CAIN (c. 1575 - c. 1650), herald bard
- SIÔN CEIRIOG - see EDWARDS, JOHN
- SIÔN CEIRIOG - see JONES, JOHN CAIN
- SION CENT (1367? - 1430?), poet
- SION CERI (fl. 1500?-1530?), poet
- SION CLYWEDOG (fl. c. 1610-1630), poet
- Siôn Conwy (c.1545 - 1606), high sheriff - see CONWAY
- SIÔN DAFYDD - see DAVIES, JOHN
- SIÔN DAFYDD - see RHYS, JOHN DAVID
- SION DAFYDD ap SIENCYN, poet
- SIÔN DAFYDD BERSON - see DAVIES, JOHN
- SIÔN DAFYDD LAS - see DAVIES, JOHN
- SIÔN DAFYDD RHYS - see RHYS, JOHN DAVID
- SION DAFYDD SIENCYN - see SION DAFYDD ap SIENCYN
- SIÔN GRYTHOR - see VAUGHAN, JOHN
- SIÔN GYMRO - see DAVIES, JOHN
- SION IEUAN CLYWEDOG - see SION CLYWEDOG
- SION LEIAF Syr (fl. c. 1480), poet and cleric
- SIÔN LLEYN - see ROBERTS, JOHN
- SION MOWDDWY (fl. c. 1575-1613), poet
- SIÔN RHYDDERCH - see RODERICK, JOHN
- SIÔN ROBERT LEWIS - see ROBERTS, JOHN
- SIÔN SINGER - see WILLIAMS, JOHN
- SION TREFOR, poet
- SIÔN TREREDYN - see EDWARDS, JOHN
- SION TUDUR (d. 1602), poet
- SIÔN WYN O EIFION - see THOMAS, JOHN
- SIÔN Y POTIAU - see EDWARDS, JOHN
- SKAIFE, Sir ERIC OMMANNEY (1884 - 1956), brigadier and patron of Welsh culture
- SKEEL, CAROLINE ANNE JAMES (1872 - 1951), historian
- SKENE, WILLIAM FORBES (1809 - 1892), Scottish historian and Celtic scholar
- SLINGSBY-JENKINS, THOMAS DAVID (1872 - 1955), secretary of a shipping company and philanthropist
- SMITH, THOMAS ASSHETON (1752 - 1828) Vaenol, Bangor, landed proprietor and quarry owner
- SMITH, WILLIAM HENRY (1894 - 1968), president of the Welsh National Opera Company
- SMYTH, ROGER (1541 - 1625), Roman Catholic priest and Welsh translator
- SNELL, DAVID JOHN (1880 - 1957), music publisher
- SOMERSET family Raglan, Troy, Crickhowell, Badminton, Chepstow
- SOMERSET, FITZROY RICHARD (4th BARON RAGLAN), (1885 - 1964), soldier, anthropologist, author
- SORANUS - see WILLIAMS, THOMAS
- SOSKICE, FRANK (Baron Stow Hill of Newport), (1902 - 1979), barrister and Labour politician
- SOULSBY, Sir LLEWELLYN THOMAS GORDON (1885 - 1966), naval architect
- SOUTHALL, JOHN EDWARD (1855 - 1928), printer, publisher, author; a member of the Society of Friends
- SOUTHALL, REGINALD BRADBURY (1900 - 1965), oil refinery director
- SPARK, THOMAS (1655 - 1692), cleric and classical scholar
- SPARKS, JOHN (1726 - 1769), early Moravian
- SPEED, GARY ANDREW (1969 - 2011), footballer
- SPINTHER - see JAMES, JOHN SPINTHER
- SPOONER, JAMES (1789 - 1856), railway engineer