Files with dem file extension are typically found as demo files recorded by various computer games (DOTA 2, Counter Strike, Quake, Hexen, Unreal Tournament etc.).
.DEM file extension - Source engine game demo replay
What is dem file? How to open dem files?
File type specification:
File extension dem is most likely best known for its use for files created when recording matches in computer game Counter Strike (or other Half-Life clones) and DOTA, DOTA 2. These records are often shared between users and watches as replays.
DEM format is the default replay demo format used in Valve's Source engine.
The very same dem suffix was also used in old Quake/Hexen II computer game for recorded games, that could be re-played in the game. In case of multi-level record, the game saved the file with pak file extension.
Also, in some computer games like Unreal Tournament, Tom Clancy's Rainbow Six 3: Raven Shield game, or the the classic Descent, there were some files with file extension DEM, that contained demonstration of the game usually recorded within the game.
The default software associated to open dem file:
Company or developer:
Valve Corporation
Dota 2 is successor of Defense of the Ancients (DotA) video game. It is a competitive game of action and strategy, played both professionally and casually by millions of passionate fans worldwide.
Players pick from a pool of over a hundred heroes, forming two teams of five players. Radiant heroes then battle their Dire counterparts to control a gorgeous fantasy landscape, waging campaigns of cunning, stealth, and outright warfare.
DotA (Defense of the Ancients) 
Company or developer:
PlayDotA.com
DotA is a competitive game of action and strategy, played both professionally and casually by millions of passionate fans worldwide. Players pick from a pool of over a hundred heroes, forming two teams of five players.
List of software applications associated to the .dem file extension
Recommended software programs are sorted by OS platform (Windows, macOS, Linux, iOS, Android etc.)
and possible program actions that can be done with the file: like open dem file, edit dem file, convert dem file, view dem file, play dem file etc. (if exist software for corresponding action in File-Extensions.org's database).
Hint:
Click on the tab below to simply browse between the application actions, to quickly get a list of recommended software, which is able to perform the specified software action, such as opening, editing or converting dem files.
dem file players - programs that can play dem file - Source engine game demo replay
Programs supporting the exension dem on the main platforms Windows, Mac, Linux or mobile. Click on the link to get more information about listed programs for play dem file action.
Microsoft Windows:
Multiplatform:
Main software associated with dem file by default:
Dota 2
DotA (Defense of the Ancients)
