| 1991 ATP Championship Series, Single Week | |
| Duration: | March 4 – November 3 |
| Edition: | 2nd |
| Tournaments: | 9 |
| Most Tournament Titles: |  Jim Courier Guy Forget (2) |
| Most Tournament Finals: | Guy Forget (3) |
| Previous: | 1990 |
| Next: | 1992 |
See main article: 1991 ATP Tour. The 1991 ATP Championship Series, Single Week was a series of tennis tournament that was part of the 1991 ATP Tour, the elite tour for professional men's tennis organised by the Association of Tennis Professionals. It formed the tier below the Grand Slam tournaments.