A list of bosses by level that have been shown to be currently implemented in the game. Mini-Bosses are marked with M.
World 1[]
1-3: Mouser M
1-GH1: Big Boo Boss
1-F1: Goomba King
1-S1: Birdo M
1-S2: Radonkel the Crow
1-6: Submarine Eggman II (World 7 Exit)
1-F2: King Totomesu
1-9: Dragonzamasu
1-F3: Pako The Octopus, Dive Man (World 3 Exit)
1-S5: Spiked Koopa
1-10: Kracko (World 8 Exit)
1-F4: Lakithunder
1-F5: Reznor
1-19: Siren
1-F6: Ghostly Mouser and Death
1-20: Boom-Boom
1-F7: Boom-Boom
1-F8: Dry Bowser
1-WB: Bowser
World 2[]
2-3: Foreman Spike M
2-4: Willy
2-S1: Zombie Horde M, Tyrant
2-10: Golden Pharaoh
2-15: Duke of India
2-16: Shoe
2-17: Dr. Cortex
2-18: Lu Bu
2-WB: General Morden
World 3[]
3-1: Guts Man
3-2: Wood Man
3-3: Cut Man
3-4: Dust Man M, Needle Man
3-5: Skull Man
3-6: Snake Man
3-7: Aqua Man, Pirate Man
3-8: Gemini Man
3-9: Bubble Man
3-10: Elec Man
3-11: Air Man
3-S2: Uranus, Galaxy Man
3-12: Slash Man
3-13: Napalm Man
3-14: VAN Pookin M, Shade Man
3-15: Knight Man
3-16: Tomahawk Man M, Centaur Man M, Charge Man
3-17: Bass
3-18: Sisi Truck M, Mad Grinder M, Nitro Man and Turbo Man
3-WB1: Top Man M, Yamato Man M, Hornet Man M, Yellow Devil
3-WB2: Sword Man M, Mercury M, Enker M, Buster Rod G M, Sonic Man M, Magic Man M, Konro Man M, Commando Man M, Dr. Wily
World 4[]
4-4: Sir Grakul
4-8: Cyberdemon
4-10: Necromancer
4-11: Draconian
4-12: Black Dragon
4-S3: Shambler
4-15: Richter Belmont
World 5[]
5-1: Clawgrip M
5-4: Octeyes
5-8: Siren (2nd Fight)
5-S1: Alien Hatchery M, The Synomium Device
5-WB: Cacobirdo
World 6[]
6-F1: Meg
6-S2: Green Walrus
6-F2: Flare Dancer
6-F3: Angler Fish
6-F4: Tryclyde
6-S4: General Guy
6-15: Kaiser Dragon
6-S5: Helmethead, Daos
6-S6: ???
World 7[]
7-1: Egg Tumbler
7-2: Egg Marine
7-4: Screw Mobile
7-8: Calliope the Clown
7-9: Ghost Snowman
7-11: Ograoum Papas
7-15: E.G.G. Truck and Metal Sonic
7-16: Egg Shroom
7-18: Silver Sonic
7-WB: Mecha Sonic M, Dr. Eggman
World 8[]
8-4: King Watinga
8-5: Whispy Woods, King Dedede (Secret fight)
8-6: Mad Scientstein
8-8: Lololo and Lalala
8-S2: King Egger
8-15: IceFoot
8-16: Thief
8-17: Pac-Chomp
8-S5: Evil Otto
8-WB: Retro Bowser M, Master Hand
World 9[]
9-3: Gomander
9-S1: WALDO
9-5: Troop of Cannons
9-6: Bacterion
9-S2: Flood Cacobirdo
9-9: Star Man
9-10: Satellite M, Bomb Man M, Robo-Corpse
9-WB: Zeta Metroid
9-?: Dobkeratops
World 0[]
0-WB: Abomination