| Competition: | OB I |
| Season: | 1921 |
| Winners: | Ferencváros |
| Prevseason: | 1920 |
| Nextseason: | 1922 |
1921 Országos Bajnokság I (men's water polo) was the 15th water polo championship in Hungary. There were five teams who played one round match for the title.
| Team | M | W | D | L | G+ | G− | P | |
|---|---|---|---|---|---|---|---|---|
| 1. | Ferencvárosi TC | 4 | 4 | 0 | 0 | 25 | 3 | 8 |
| 2. | III. ker. TVE | 4 | 3 | 0 | 1 | 11 | 7 | 6 |
| 3. | MAC | 4 | 2 | 0 | 2 | 17 | 7 | 4 |
| 4. | MAFC | 4 | 1 | 0 | 3 | 12 | 13 | 2 |
| 5. | Nemzeti SC | 4 | 0 | 0 | 4 | 1 | 36 | 0 |
* M: Matches W: Win D: Drawn L: Lost G+: Goals earned G−: Goals got P: Point