Games like Synonym
Games like Synonym include Baba Is You, LIMBO, Hollow Knight: Silksong, Return of the Obra Dinn, and The Witness. We matched them by the tags players give Synonym on Steam (Indie, Puzzle, Difficult, Sokoban, and Atmospheric): closest matches first, and among equally close ones the better-known games.

Baba Is You
Shared: Indie · Puzzle · Difficult

LIMBO
Shared: Indie · Puzzle · Atmospheric

Hollow Knight: Silksong
Shared: Indie · Difficult · Singleplayer

Return of the Obra Dinn
Shared: Puzzle · Indie · Atmospheric

The Witness
Shared: Puzzle · Indie · Singleplayer

Hollow Knight
Shared: Difficult · Indie · Singleplayer

Chants of Sennaar
Shared: Puzzle · Indie · Singleplayer

Papers, Please
Shared: Indie · Puzzle · Singleplayer

Antichamber
Shared: Puzzle · Indie · Singleplayer

Void Stranger
Shared: Puzzle · Sokoban · Indie

Firework
Shared: Indie · Puzzle · Atmospheric

Gorogoa
Shared: Puzzle · Indie · Atmospheric

Patrick's Parabox
Shared: Puzzle · Sokoban · Indie

Stephen's Sausage Roll
Shared: Indie · Puzzle · Difficult

The Room
Shared: Puzzle · Atmospheric · Indie

The Last Campfire
Shared: Puzzle · Indie · Singleplayer

Little Nightmares
Shared: Atmospheric · Puzzle · Indie

The Talos Principle
Shared: Puzzle · Atmospheric · Singleplayer

OneShot
Shared: Indie · Puzzle · Atmospheric

Rain World
Shared: Difficult · Indie · Atmospheric

The Binding of Isaac: Rebirth
Shared: Indie · Difficult · Singleplayer

Seen
Shared: Indie · Puzzle · Singleplayer

Stray
Shared: Atmospheric · Singleplayer · Puzzle

FAR: Lone Sails
Shared: Atmospheric · Indie · Singleplayer