| Prevseason Year: | 1988–89 |
| Season Champs: | Porin Ässät |
| Second Place: | Hockey-Reipas |
| Promote To: | SM-liiga |
| Promote: | Porin Ässät and Hockey-Reipas |
| Relegate To: | II-Divisioona |
| Relegate: | Hokki-Salo and KooVee |
| Prevseason Link: | 1988–89 I-Divisioona season |
| Nextseason Link: | 1990–91 I-Divisioona season |
| Nextseason Year: | 1990–91 |
The 1989–90 I-Divisioona season was the 16th season of the I-Divisioona, the second level of Finnish ice hockey. 12 teams participated in the league, and Ässät Pori won the championship. Ässät Pori and Hockey-Reipas Lahti qualified for the promotion/relegation round of the SM-liiga.
| Club | GP | W | T | L | GF–GA | Pts | ||
|---|---|---|---|---|---|---|---|---|
| 1. | 44 | 37 | 3 | 4 | 355:144 | 77 | ||
| 2. | 44 | 31 | 3 | 10 | 246:133 | 65 | ||
| 3. | 44 | 30 | 3 | 11 | 255:141 | 63 | ||
| 4. | 44 | 28 | 2 | 14 | 279:179 | 58 | ||
| 5. | 44 | 20 | 3 | 21 | 229:199 | 43 | ||
| 6. | 44 | 19 | 5 | 20 | 213:236 | 43 | ||
| 7. | 44 | 20 | 2 | 22 | 190:208 | 42 | ||
| 8. | 44 | 17 | 2 | 25 | 192:257 | 36 | ||
| 9. | 44 | 17 | 1 | 26 | 214:229 | 35 | ||
| 10. | 44 | 12 | 3 | 29 | 189:326 | 27 | ||
| 11. | 44 | 10 | 1 | 33 | 166:299 | 21 | ||
| 12. | 44 | 9 | 0 | 35 | 169:346 | 18 |