Games like Psychomachia
Games like Psychomachia include Mouthwashing, Layers of Fear (2016), SOMA, Outlast, and Still Wakes the Deep. We matched them by the tags players give Psychomachia on Steam (Psychological Horror, Horror, Psychological, Walking Simulator, and Psychedelic): closest matches first, and among equally close ones the better-known games.

Mouthwashing
Shared: Psychological Horror · Horror · Psychological

Layers of Fear (2016)
Shared: Psychological Horror · Horror · Walking Simulator

SOMA
Shared: Psychological Horror · Horror · Atmospheric

Outlast
Shared: Horror · Psychological Horror · Atmospheric

Still Wakes the Deep
Shared: Psychological Horror · Horror · Walking Simulator

Outlast 2
Shared: Psychological Horror · Horror · Dark

No, I'm not a Human
Shared: Psychological Horror · Horror · Indie

Amnesia: The Dark Descent
Shared: Horror · Psychological Horror · Gore

Blair Witch
Shared: Horror · Psychological Horror · Walking Simulator

Buckshot Roulette
Shared: Horror · Dark · Psychological

Visage
Shared: Psychological Horror · Horror · Dark

Phasmophobia
Shared: Horror · Psychological Horror · Supernatural

Layers of Fear 2 (2019)
Shared: Psychological Horror · Horror · Walking Simulator

Scorn
Shared: Horror · Psychological Horror · Gore

Hellblade: Senua's Sacrifice
Shared: Psychological · Atmospheric · Horror

Fran Bow
Shared: Psychological Horror · Horror · Dark

The Park®
Shared: Psychological Horror · Horror · Walking Simulator

Amnesia: A Machine For Pigs
Shared: Horror · Walking Simulator · Psychological Horror

From The Darkness
Shared: Psychological Horror · Horror · Dark

What Remains of Edith Finch
Shared: Walking Simulator · Atmospheric · Psychological Horror

The Medium
Shared: Horror · Psychological Horror · Atmospheric

God's Basement
Shared: Psychological Horror · Horror · Walking Simulator

Welcome to Kowloon
Shared: Horror · Psychological Horror · Walking Simulator

Sally Face - Episode One
Shared: Psychological Horror · Horror · Dark