Open the game's executable (ShippingPC-BmGame.exe) with a hex editor and look for this string : 25 00 00 08 00 31 86 80.
Replace the 25 with 0D, and the 31 with 89. Save the file, launch the game, go to the challenge mode, you should have a new Joker mode available to play. Enjoy.

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Alternate method : download and install this.

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EDIT : sorry, it seems that this trick only works with the Steam / Epic version, not the GOG version.

EDIT 2 : according to Rubercaine, it works with the GOG version if you do the change at the offset 012153C0.

It's not available in the game itself to unlock anywhere?

It's specific to PS3. The Playstation line has other exclusives too. In Arkham Knight PS4 players get the 1960 Batman skin, as well as the Justice League 3000 skin. There is also a tank challenge with Scarecrow.

I see, thanks. Weird that it's not standard in the PC version. Would have though any exclusivity would have run out a long time ago...

That might be the case, maybe they just never bothered to update the PC version, there is little money to be made, espescially if people the content for free.
In other games (like the Lego DC games) PS exclusives were sold later as separate DLC, not in cluded in any Season Pack/Deluxe Edition/whatever.

I would pay for the exclusive skins and other extra content.

Same, I did so occasionally. But in the end it's their decision.
The Sony exclusve DLCs are a PITA in every game.

Sinon, t'as aussi un pack de cartes et autres à DL puis installer.
Tu trouveras ça ici :

[url=https://www.pcgamingwiki.com/wiki/Batman:_Arkham_Asylum#Ex-exclusive_PS3_DLCs]https://www.pcgamingwiki.com/wiki/Batman:_Arkham_Asylum#Ex-exclusive_PS3_DLCs[/url]

À voir si c'est compatible version GOG.

J'ai déjà mis le lien. ;)

Autant pour moi.
J'avais pas vu.

i downloaded the file and put it into the game folder but the joker isnt there , anyone else have that issue?, am i doing something wrong?

Same here. The DLC maps like Scarecrow's map or Heart of Darkness are visible, so some DLC was activated, but Joker isn't available in the character selection menu.

(also, I tried a Hex editor, but the string I was supposed to look for wasn't present in the exe-file)

It works, actually. The string is located on the offset 012153C0, not 012153C6. Try it.

I don't get it! What do you mean by offset? Which/what FULL string am I looking/searching for?
Which values/numbers do I replace? What values do I put in their place?

You will need to download a hex editing program such as HxD.

Once you've installed HxD, open the program.

Then select file --> open and then go to C:\GOG Games\Batman Arkham Asylum GOTY\Binaries or wherever you installed the game and go to the binaries folder.

Select the ShippingPC-BmGame.exe file.

It will load the file into the hex editor and you'll see a screen full of numbers. The numbers on the left side are the offset.

Scroll down until the number on the left gets to 012153C0.

Look over to the right and you will see a bunch of numbers on that line. 33 86 80 06 00 00 25 00 00 08 00 31 86 80 06 00



Click on the 25 and change it to 0D.
Then click on the 31 and change it to 89.

Save the changed file and you should be good to go.