- ILES, MARTHA (d. 1825), heiress - see LLOYD
- ILLINGWORTH, LESLIE GILBERT (1902 - 1979), political cartoonist
- ILLTUD (c. 475 - c. 525), Celtic saint and one of the founders of monachism in Britain
- INCO BRYDYDD (fl. c. 1480), poet
- INDEX - see WILLIAMS, DAFYDD RHYS
- INGRAM, JAMES (d. 1788), Methodist exhorter, and afterwards Independent minister
- INNES, JAMES DICKSON (1887 - 1914), artist
- INNES, JOHN (1853? - 1923), accountant and antiquary
- INSOLE, GEORGE (1790 - 1851), colliery proprietor
- INSOLE, JAMES HARVEY (1821 - 1901), colliery proprietor
- IOAN ab HU FEDDYG - see PUGHE, JOHN
- IOAN ab HYWEL - see HOWELL, JOHN
- IOAN ap IAGO - see JAMES, JOHN
- IOAN ap IOAN - see WILLIAMS, JOHN
- IOAN ARFON - see GRIFFITH, JOHN OWEN
- IOAN BROTHEN - see JONES, JOHN
- IOAN BRYNGWYN BACH - see JONES, JOHN
- IOAN EMLYN - see JONES, JOHN EMLYN
- IOAN GLAN DYFRDWY - see PAGE, JOHN
- IOAN GLANDYFROEDD - see HOWELL, JOHN
- IOAN IDRIS - see DAVIES, JOHN IDRIS
- IOAN MADOG - see WILLIAMS, JOHN
- IOAN MAESGRUG - see JONES, JOHN EDWARD
- IOAN MAI - see WILLIAMS, JOHN
- IOAN MEIRION - see JAMES, JOHN
- IOAN PEDR - see PETER, JOHN
- IOAN RHAGFYR - see WILLIAMS, JOHN
- IOAN SIENGCIN - see JENKIN, JOHN
- IOAN TACHWEDD - see EVANS, JOHN
- IOAN TEGID - see JONES, JOHN
- IOANNES PHILIPPI - see PUW
- IOCYN DDU - see RICHARDS, JOHN
- IOCYN DDU ab ITHEL GRACH (fl. c. 1380), poet
- IOHANNES GALLENSIS - see IOHANNES WALLENSIS
- IOHANNES WALEYS - see IOHANNES WALLENSIS
- IOLO CAERNARFON - see ROBERTS, JOHN JOHN
- IOLO GOCH (c. 1325 - c. 1400), poet
- IOLO GOCH (c. 1320 - c. 1398), poet
- IOLO MORGANWG - see WILLIAMS, EDWARD
- IOLO TREFALDWYN - see DAVIES, EDWARD
- IONORON GLAN DWYRYD - see WALTER, ROWLAND
- IORTHRYN GWYNEDD - see THOMAS, ROBERT DAVID
- IORWERTH, abbot of Talley and bishop of S. Davids
- IORWERTH ab OWAIN, Lord of Caerleon - see MORGAN ap HYWEL
- IORWERTH ab Y CYRIOG (fl. c. 1380), poet
- IORWERTH ap BLEDDYN (d. 1111), prince of Powys
- IORWERTH ap MADOG (fl. 1240?-1268?), jurist
- IORWERTH BELI (fl. early in the 14th century) Gwynedd, poet
- IORWERTH CEITHO - see JONES, EDWARD
- IORWERTH DRWYNDWN (d. c. 1174), prince of Gwynedd