Games like Summer Nightmare
Games like Summer Nightmare include Escape the Backrooms, Outlast, Five Nights at Freddy's: Security Breach, MiSide, and SILENT HILL: Townfall. We matched them by the tags players give Summer Nightmare on Steam (Adventure, Action, 3D, First-Person, and Psychedelic): closest matches first, and among equally close ones the better-known games.

Escape the Backrooms
Shared: Horror · First-Person · Psychological Horror

Outlast
Shared: First-Person · Horror · Psychological Horror

Five Nights at Freddy's: Security Breach
Shared: First-Person · Horror · Singleplayer

MiSide
Shared: Psychological Horror · First-Person · 3D

SILENT HILL: Townfall
Shared: Adventure · Action · First-Person

The Beast Inside
Shared: Adventure · First-Person · Action

Five Nights at Freddy's: Secret of the Mimic
Shared: Adventure · First-Person · Action

Resident Evil Village
Shared: First-Person · Action · Horror

Resident Evil Requiem
Shared: Action · First-Person · Horror

Poppy Playtime
Shared: First-Person · Horror · Singleplayer

Resident Evil 7 Biohazard
Shared: First-Person · Horror · Action

Grand Theft Auto V Legacy
Shared: Action · Adventure · First-Person

Garten of Banban
Shared: First-Person · Horror · Psychological Horror

Phasmophobia
Shared: Horror · Psychological Horror · First-Person

The Forest
Shared: Adventure · Horror · Action

Bendy and the Dark Revival
Shared: Action · First-Person · Adventure

We Happy Few
Shared: Adventure · Action · First-Person

Golden Light
Shared: Action · 3D · Adventure

Outlast 2
Shared: First-Person · Psychological Horror · Horror

Dark Deception
Shared: First-Person · Horror · Psychological Horror

Atomic Heart
Shared: Adventure · Action · Singleplayer

Ready or Not
Shared: First-Person · Action · Singleplayer

BioShock Infinite
Shared: Action · First-Person · Adventure

Thief
Shared: Action · Adventure · First-Person