| Club: | Millwall |
| Season: | 2009–10 |
| Manager: | Kenny Jackett |
| Chairman: | John Berylson |
| League: | League One |
| League Result: | 3rd (promoted via play-offs) |
| Cup1: | FA Cup |
| Cup1 Result: | 2nd round |
| Cup2: | Carling Cup |
| Cup2 Result: | 3rd round |
| League Topscorer: | Steve Morison (21) |
| Season Topscorer: | Steve Morison (23) |
| Highest Attendance: | 17,632 vs Charlton Athletic (13 March 2010) |
| Lowest Attendance: | 6,617 vs Yeovil Town (29 September 2009) |
| Average Attendance: | 10,834 |
| Pattern B1: | _white_right_shoulder |
| Leftarm1: | 0033FF |
| Body1: | 0033FF |
| Rightarm1: | 0033FF |
| Shorts1: | 0033FF |
| Socks1: | 0033FF |
| Pattern La2: | _blackshoulders |
| Pattern B2: | _blackstripes |
| Pattern Ra2: | _blackshoulders |
| Leftarm2: | FF0000 |
| Body2: | FF0000 |
| Rightarm2: | FF0000 |
| Shorts2: | 000000 |
| Socks2: | 000000 |
| Prevseason: | 2008–09 |
| Nextseason: | 2010–11 |
The 2009–10 Football League One season saw Millwall promoted back to The Championship via the Play-offs, after finishing 3rd in the table. Millwall celebrated their 125th anniversary this season. It was the club's 84th continuous season in the Football League and 41st in the third tier.