| Name | CR | Type | Environment |
| Jabberwock | 23 | dragon (air, fire) | any forests |
| Jackal | 1/2 | animal | warm deserts or plains |
| Jackalwere | 2 | magical beast (shapechanger) | warm deserts |
| Jack-o'-Lantern | 1 | plant | temperate forests, hills, or plains (pumpkin patches) |
| Jellyfish, Bathysphere | 1/2 | vermin (aquatic, extraplanar) | any water (Plane of Water) |
| Jellyfish, Death’s Head Jellyfish | 1 | vermin (aquatic) | any oceans |
| Jellyfish, Giant Jellyfish | 7 | vermin (aquatic) | any oceans |
| Jellyfish, Jellyfish Swarm | 6 | vermin (aquatic, swarm) | any aquatic |
| Jellyfish, Sapphire Jellyfish | 11 | vermin (aquatic) | warm or temperate oceans |
| Jinmenju | 11 | plant | temperate hills or mountains |
| Jorogumo | 12 | monstrous humanoid (shapechanger) | temperate mountain valleys |
| Joyful Thing | 6 | outsider (native) | any |
| Jubjub Bird | 15 | magical beast | temperate or warm forests |
| Juggerloathe | 16 | magical beast | any (Hold of Belkzen) |
| Juggernaut | 11 | construct | any |
| Jyoti | 9 | outsider (extraplanar) | any (Positive Energy Plane) |