Games like Somnium
Games like Somnium include Portal 2, Supraland, Slime Rancher, Supraland Six Inches Under, and Viewfinder. We matched them by the tags players give Somnium on Steam (Adventure, Action-Adventure, Puzzle, Platformer, and Dragons): closest matches first, and among equally close ones the better-known games.

Portal 2
Shared: Puzzle · Platformer · First-Person

Supraland
Shared: Adventure · Exploration · Puzzle Platformer

Slime Rancher
Shared: Adventure · Exploration · Cute

Supraland Six Inches Under
Shared: Adventure · Puzzle · Exploration

Viewfinder
Shared: Puzzle · Adventure · Puzzle Platformer

ANIMAL WELL
Shared: Puzzle · Exploration · Platformer

Chained Together
Shared: Adventure · Platformer · Action-Adventure

FEZ
Shared: Puzzle · Platformer · Adventure

Ori and the Will of the Wisps
Shared: Platformer · Adventure · Action-Adventure

Indiana Jones and the Great Circle
Shared: Action-Adventure · Puzzle · Adventure

Supraworld
Shared: Puzzle · Exploration · Adventure

TUNIC
Shared: Puzzle · Exploration · Action-Adventure

Atomic Heart
Shared: Adventure · Action-Adventure · FPS

LIMBO
Shared: Puzzle · Platformer · Adventure

Trine 2: Complete Story
Shared: Action-Adventure · Platformer · Puzzle

Far Cry® 5
Shared: Action-Adventure · FPS · Exploration

COCOON
Shared: Adventure · Puzzle · Exploration

Darwin's Paradox!
Shared: Adventure · Action-Adventure · Platformer

Outer Wilds
Shared: Adventure · Exploration · Puzzle

The Witness
Shared: Puzzle · Exploration · Adventure

Trine 3: The Artifacts of Power
Shared: Action-Adventure · Puzzle Platformer · Puzzle

Amnesia: Rebirth
Shared: Action-Adventure · Puzzle · Adventure

Murders on the Yangtze River
Shared: Adventure · Puzzle · Exploration

Chants of Sennaar
Shared: Adventure · Puzzle · Colorful