- SAUNDERSON, ROBERT (1780 - 1863), printer and publisher
- SAUNDERSON, ROBERT (1814), printer and publisher - see SAUNDERSON, ROBERT
- SAYCE, GEORGE ETHELBERT (1875 - 1953), journalist and newspaper proprietor
- SBARDUN - see HUWS, ALUN ‘SBARDUN’
- SCARROTT, JOHN (1870 - 1947), boxing promoter
- SCHREIBER, BERTIE - see GUEST, Lady CHARLOTTE ELIZABETH
- SCHREIBER, Lady CHARLOTTE ELIZABETH - see GUEST, Lady CHARLOTTE ELIZABETH
- SCORPION - see ROBERTS, THOMAS
- SCOTT-ELLIS, THOMAS EVELYN (8th BARON HOWARD DE WALDEN, 4th BARON SEAFORD), (1880 - 1946), landowner and sportsman, writer, and patron of the arts
- SCOURFIELD, Sir JOHN HENRY (1808 - 1876), author
- SCUDAMORE family
- SEABORNE-DAVIES, DAVID RICHARD (1904 - 1984), lawyer and politician
- SEAFORD, 4th Baron - see SCOTT-ELLIS, THOMAS EVELYN
- SEAGER, GEORGE LEIGHTON (BARON LEIGHTON of St. Mellons), (1896 - 1963), merchant and shipowner
- SEAGER, JOHN ELLIOT (1891 - 1955), shipowner
- SEEBOHM, FREDERIC (1833 - 1912), historian and banker
- SEFNYN (fl. second half of the 14th century), poet
- SEIRIOL (fl. c. 500- c. 550), founder and first abbot of Penmon church
- SEISYLL ap CLYDOG (fl. 730), king of the combined realm of Ceredigion and Ystrad Tywi
- SEISYLL BRYFFWRCH (1155 - 1175), poet
- SEYLER, CLARENCE ARTHUR (1866 - 1959), chemist and public analyst
- SGORFAI - see WILLIAMS, JOHN HUW
- SHADRACH, AZARIAH (1774 - 1844), schoolmaster, Independent minister, and author
- SHAND, FRANCES BATTY (c.1815 - 1885), charity worker
- SHANKLAND, THOMAS (1858 - 1927), bibliophile and historian
- SHEEN, ALFRED WILLIAM (1869 - 1945), surgeon and first Provost of the Welsh National School of Medicine
- SHEEN, THOMAS (1718 - 1790), Methodist exhorter, afterwards an Antinomian
- SHEPHERD, DONALD JOHN (1927 - 2018), cricketer
- SHEPPARD, ARNOLD ALONZO (1908 - 1979), boxer
- SHIPLEY, WILLIAM DAVIES (1745 - 1826), cleric
- SHONI SGUBOR FAWR - see JONES, JOHN
- SHONI SGUBORFAWR - see JONES, JOHN
- SHORT, THOMAS VOWLER (1790 - 1872), bishop of St Asaph
- SIAMAS GWYNEDD - see CHARLES, EDWARD
- SIAMAS WYNEDD - see CHARLES, EDWARD
- SIANCYN FYNGLWYD (fl. c. 1470), poet
- SIARL WYN O BENLLYN (1810? - 1832), poet - see SAUNDERSON, ROBERT
- SIBLY, Sir THOMAS FRANKLIN (1883 - 1948), geologist and university administrator
- SIDDONS, SARAH (1755 - 1831), actress
- SIDNEY, Sir HENRY (1529 - 1586) Penshurst, Kent, president of Wales
- SIEFFRE - see GEOFFREY of MONMOUTH
- SIENCYN PEN-HYDD - see THOMAS, JENKIN
- SILS ap SION (fl. end of the 16th century), bard
- SILURIST - see VAUGHAN, HENRY
- SILVERTHORNE, THORA (1910 - 1999), nurse and trade unionist
- SIMMONS, JOSEPH (1694? - 1774), Independent minister, and schoolmaster
- SIMON of Stackpole Elidor, 1st Viscount - see SIMON, JOHN ALLSEBROOK
- SIMON, BEN (c. 1703 - 1793), dissenter and copyist
- SIMON, JOHN ALLSEBROOK (1st VISCOUNT SIMON of Stackpole Elidor), (1873 - 1954), judge and politician
- SIMONS, JOSEPH - see SIMMONS, JOSEPH