Games like Boomsday
Games like Boomsday include Hades, Hades II, Have a Nice Death, Megabonk, and Curse of the Dead Gods. We matched them by the tags players give Boomsday on Steam (Action, Arena Shooter, Isometric, Destruction, and Combat): closest matches first, and among equally close ones the better-known games.

Hades
Shared: Action · Action Roguelike · Roguelike

Hades II
Shared: Action · Roguelike · Action Roguelike

Have a Nice Death
Shared: Action · Action Roguelike · Roguelike

Megabonk
Shared: Action Roguelike · Roguelike · Action

Curse of the Dead Gods
Shared: Isometric · Action · Action Roguelike

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

THE FINALS
Shared: Destruction · Action · Arena Shooter

Risk of Rain 2
Shared: Action Roguelike · Action · Roguelike

Wedding Witch
Shared: Action · Action Roguelike · Roguelike

The Binding of Isaac
Shared: Action Roguelike · Roguelike · Action

Sephiria
Shared: Action Roguelike · Action · Roguelike

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

Arms of God
Shared: Arena Shooter · Action Roguelike · Action

Crab Champions
Shared: Action Roguelike · Roguelike · 3D

Cinderia
Shared: Action · Action Roguelike · Roguelike

Brotato
Shared: Roguelike · Action Roguelike · Arena Shooter

Stick It to the Stickman
Shared: Action · Roguelike · Combat

Atomicrops
Shared: Action · Roguelike · Action Roguelike

Ravenswatch
Shared: Action · Roguelike · Action Roguelike

Touhou Dystopian
Shared: Action · Action Roguelike · 3D

Shotgun Farmers
Shared: Action · Arena Shooter · Colorful

Windblown
Shared: Action Roguelike · Combat · Action

DR LIVESEY ROM AND DEATH EDITION
Shared: Action · Roguelike · Cartoon

MADNESS: Project Nexus
Shared: Arena Shooter · Action · Combat