- inexorable: unstoppable
- infamous: notorious, shocking
- infinitesimal: tiny, minute, minuscule
- ingenious: brilliant, imaginative, clever
- ingenuous: naive, trusting, gullible
- inhibit: control, limit, restrain
- innate: natural, inborn
- innocuous: harmless, inofensive
- intense: powerful, heightened, concentrated
- intricate: complicated, complex, involved
- irate: angry, annoyed, upset
- jagged: rough, rugged, uneven, irregular
- jeopardy: danger, hazard, threat
- jolly: joyful, happy, cheerful
- jolt: shock, jar, shake up, surprise
- keen: sharp, shrewd, clever, bright, eager, enthusiastic
- knack: skill, ability, aptitude, talent