Games like Ninja Monkey
Games like Ninja Monkey include Vampire Survivors, Megabonk, Halls of Torment, Death Must Die, and Brotato. We matched them by the tags players give Ninja Monkey on Steam (Action Roguelike, Bullet Hell, Survival, Action, and Roguelite): closest matches first, and among equally close ones the better-known games.

Vampire Survivors
Shared: Action Roguelike · Bullet Hell · Roguelite

Megabonk
Shared: Action Roguelike · Bullet Hell · Roguelite

Halls of Torment
Shared: Action Roguelike · Bullet Hell · Roguelite

Death Must Die
Shared: Action Roguelike · Bullet Hell · Roguelite

Brotato
Shared: Bullet Hell · Action Roguelike · Roguelike

20 Minutes Till Dawn
Shared: Action Roguelike · Bullet Hell · Survival

Skul: The Hero Slayer
Shared: Action Roguelike · Action · Pixel Graphics

Enter the Gungeon
Shared: Action Roguelike · Bullet Hell · Pixel Graphics

Hades
Shared: Action Roguelike · Roguelite · Action

Nuclear Throne
Shared: Action Roguelike · Bullet Hell · Pixel Graphics

BALL x PIT
Shared: Action · Bullet Hell · Roguelite

Atomicrops
Shared: Bullet Hell · Action · Action Roguelike

Dungreed
Shared: Action Roguelike · Action · Roguelite

Everything is Crab: The Animal Evolution Roguelite
Shared: Action Roguelike · Roguelite · Survival

Noita
Shared: Pixel Graphics · Roguelike · Action Roguelike

The Binding of Isaac: Rebirth
Shared: Action Roguelike · Roguelike · Pixel Graphics

Soulstone Survivors
Shared: Action Roguelike · Bullet Hell · Roguelite

Hades II
Shared: Action · Action Roguelike · Roguelite

暖雪 Warm Snow
Shared: Action Roguelike · Roguelite · Roguelike

Streets of Rogue
Shared: Action Roguelike · Roguelite · Pixel Graphics

Boneraiser Minions
Shared: Action Roguelike · Bullet Hell · Roguelite

Risk of Rain (2013)
Shared: Action Roguelike · Action · Roguelike

Dead Cells
Shared: Action Roguelike · Roguelike · Roguelite

Tower of Babel: Survivors of Chaos
Shared: Action Roguelike · Bullet Hell · Roguelite