The 23-year-old had three years remaining on the deal he signed on joining from Peterborough United in 2014 but will now remain with the Reds until 2021.
Assombalonga, who missed much of the 2015-16 season with a serious knee injury, has scored 19 goals in 39 games for Forest.
His three goals this season have helped the East Midlands club to seventh place in the Championship after five games.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| C | Manchester CityMan City | 38 | 28 | 7 | 3 | 96 | 34 | 62 | 91 |
| 2 | Arsenal | 38 | 28 | 5 | 5 | 91 | 29 | 62 | 89 |
| 3 | Liverpool | 38 | 24 | 10 | 4 | 86 | 41 | 45 | 82 |
| 4 | Aston Villa | 38 | 20 | 8 | 10 | 76 | 61 | 15 | 68 |
| 5 | Tottenham HotspurSpurs | 38 | 20 | 6 | 12 | 74 | 61 | 13 | 66 |
| 6 | Chelsea | 38 | 18 | 9 | 11 | 77 | 63 | 14 | 63 |
| 7 | Newcastle UnitedNewcastle | 38 | 18 | 6 | 14 | 85 | 62 | 23 | 60 |
| 8 | Manchester UnitedMan Utd | 38 | 18 | 6 | 14 | 57 | 58 | -1 | 60 |
| 9 | West Ham UnitedWest Ham | 38 | 14 | 10 | 14 | 60 | 74 | -14 | 52 |
| 10 | Crystal Palace | 38 | 13 | 10 | 15 | 57 | 58 | -1 | 49 |
| 11 | Brighton & Hove AlbionBrighton | 38 | 12 | 12 | 14 | 55 | 62 | -7 | 48 |
| 12 | Bournemouth | 38 | 13 | 9 | 16 | 54 | 67 | -13 | 48 |
| 13 | Fulham | 38 | 13 | 8 | 17 | 55 | 61 | -6 | 47 |
| 14 | Wolverhampton WanderersWolves | 38 | 13 | 7 | 18 | 50 | 65 | -15 | 46 |
| 15 | Everton | 38 | 13 | 9 | 16 | 40 | 51 | -11 | 40 |
| 16 | Brentford | 38 | 10 | 9 | 19 | 56 | 65 | -9 | 39 |
| 17 | Nottingham ForestNott'm Forest | 38 | 9 | 9 | 20 | 49 | 67 | -18 | 32 |
| R | Luton TownLuton | 38 | 6 | 8 | 24 | 52 | 85 | -33 | 26 |
| R | Burnley | 38 | 5 | 9 | 24 | 41 | 78 | -37 | 24 |
| R | Sheffield UnitedSheff Utd | 38 | 3 | 7 | 28 | 35 | 104 | -69 | 16 |
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