- IORWERTH FYCHAN ap IORWERTH ap ROTPERT (fl. c. 1300), poet
- IORWERTH FYNGLWYD (fl. c. 1480-1527), bard
- IORWERTH GLAN ALED - see ROBERTS, EDWARD
- IORWERTH GOES HIR - see JONES, EDWARD
- IORWERTH LLWYD ab Y GARGAM - see IEUAN LLWYD ab Y GARGAM
- IORWERTH PENTYRCH - see GITTINS, EDWARD
- IORWERTH TWROG - see JONES, JOHN EDWARD
- IRBY, GEORGE FLORANCE (1860 - 1941), landowner and scientist
- ISAAC, DANIEL (fl. end of 18th century), Baptist minister - see WINTER, CHARLES
- ISAAC, DAVID LLOYD (1818 - 1876), cleric and author
- ISAAC, EVAN (1865 - 1938), Wesleyan minister
- ISAAC, JACOB, Baptist minister - see WINTER, CHARLES
- ISAAC, JACOB - see WINTER, CHARLES
- ISALAW - see RICHARDS, JOHN
- ISFAEL (fl. 6th century), saint
- ISFOEL - see JONES
- ISGARN - see DAVIES, RICHARD
- ISHMAEL - see ISFAEL
- ISLWYN - see THOMAS, WILLIAM
- ISLWYN, Baron - see HUGHES, ROYSTON JOHN
- ISMAIL, Sheikh SAEED HASSAN (1930 - 2011), Muslim leader
- ISNANT - see HARKER, EDWARD
- ITHEL ap RHOTPERT or ROBERT (fl. 1357-1382), archdeacon
- ITHEL DDU (fl. second half of 14th century), poet
- IVON - see JONES, JOHN
- IWAN - see WILLIAMS, DAVID