Open the Save_System_Settings.json from C:\Users\[Your User]\AppData\LocalLow\Steel City Interactive\Undisputed\shared and change the values of 'width' and 'height' as desired.

Still, after three updates.... 2560x1440 is the maximum ultrawide screen resolution I can set in the options.
As the multiplayer beta supported ultrawide screen resolutions flawlessly, it really sucked that the early access version suddenly did not.


My native resolution: 3840x1600 (24:10) @144Hz
My preferred custom resolution: 4800x2000 (24:10) @120Hz
Nvidia Driver Version: 531.41
Win11 (64bit)

1. Preperations
Go to the ingame options and set everything as you prefer.
Optimally set the screen mode to fullscreen and close the game.
Hint: This needs to be done, as you don't want to change any option ingame after this fix. As it could overwrite the settings file and you'd need to do this fix again.

2. How to edit the settings file
Open the file explorer and go to
C:\Users\[Your User]\AppData\LocalLow\Steel City Interactive\Undisputed\Settings
Then open the file Video_Options.json with the text editor (notepad).

2.1 Set your ultrawide screen resolution
And just overwrite the width & height values with your prefered ultrawide screen resolution, as shown in the following screenshot:


Also check if the display mode is set to "ExclusiveFullScreen", if not do so.
And finally hit save and exit.

3. Start the game
...and now you will be able to enjoy Undisputed in all its' widescreen aspect ratio glory.

Note: Have in mind that changing any setting ingame - after this workaround - might overwrite this fix.

Here are some screenshots taken at 4800x2000 with an aspect ratio of 24:10: