Games like The Witness
Games like The Witness include Firewatch, Outer Wilds, Antichamber, Slime Rancher, and What Remains of Edith Finch. We matched them by the tags players give The Witness on Steam (Puzzle, Exploration, First-Person, Open World, and Singleplayer): closest matches first, and among equally close ones the better-known games.

Firewatch
Shared: First-Person · Exploration · Atmospheric

Outer Wilds
Shared: Exploration · Puzzle · Open World

Antichamber
Shared: Puzzle · First-Person · Exploration

Slime Rancher
Shared: Exploration · Open World · Singleplayer

What Remains of Edith Finch
Shared: First-Person · Exploration · Atmospheric

The Talos Principle
Shared: Puzzle · First-Person · Singleplayer

The Long Dark
Shared: Exploration · Open World · First-Person

Fallout 4
Shared: Open World · Exploration · Singleplayer

Big Walk
Shared: Puzzle · Exploration · Open World

Chants of Sennaar
Shared: Puzzle · Singleplayer · Colorful

INFRA
Shared: Puzzle · Exploration · First-Person

Obduction
Shared: Puzzle · Exploration · First-Person

Blue Prince
Shared: Puzzle · Exploration · Singleplayer

Stray
Shared: Exploration · Singleplayer · Puzzle

Return of the Obra Dinn
Shared: Puzzle · Singleplayer · First-Person

RiME
Shared: Puzzle · Exploration · Singleplayer

Riven
Shared: Puzzle · Exploration · First-Person

Quern - Undying Thoughts
Shared: Puzzle · Exploration · First-Person

The Room
Shared: Puzzle · Singleplayer · Atmospheric

AER Memories of Old
Shared: Exploration · Open World · Indie

Call of the Sea
Shared: Puzzle · First-Person · Exploration

Slime Rancher 2
Shared: Open World · Singleplayer · First-Person

ABZU
Shared: Exploration · Atmospheric · Singleplayer

Valley
Shared: Exploration · First-Person · Singleplayer