diff --git a/docs/directory_structure.md b/docs/directory_structure.md
index 72a62eee42..ba1b7673c8 100644
--- a/docs/directory_structure.md
+++ b/docs/directory_structure.md
@@ -38,6 +38,17 @@ your operating system:
 
     It includes the OpenTTD files (grf+lng) and it will work as long as they
     are not touched
+7. The Atari Transport Tycoon Deluxe directory, if installed (path may vary)
+
+    This refers to the `CD` folder within the Transport Tycoon Deluxe
+    installation folder (2026 Atari re-release). OpenTTD detects the presence
+    of this folder based upon the contents of the `installpath.ini` file located
+    in:
+    - Windows: `%APPDATA%\Atari\Transport Tycoon Deluxe`
+    - macOS: `~/Library/Application Support/Atari/Transport Tycoon Deluxe`
+    - Linux: `$XDG_DATA_HOME/Atari/Transport Tycoon Deluxe`
+
+    This is used only for the loading of base sets (graphics, sound, music).
 
 Different types of data or extensions go into different subdirectories of the
 chosen main OpenTTD directory:
diff --git a/src/fileio.cpp b/src/fileio.cpp
index 90d943e74a..f1fc868e08 100644
--- a/src/fileio.cpp
+++ b/src/fileio.cpp
@@ -167,6 +167,9 @@ std::string FioGetDirectory(Searchpath sp, Subdirectory subdir)
 	assert(subdir < Subdirectory::End);
 	assert(sp < Searchpath::End);
 
+	/* For official TTD directory, don't include the subdirectory. */
+	if (sp == Searchpath::TransportTycoonDeluxeDir && subdir == Subdirectory::Baseset) return _searchpaths[sp];
+
 	return fmt::format("{}{}", _searchpaths[sp], _subdirs[subdir]);
 }
 
@@ -197,7 +200,7 @@ static std::optional<FileHandle> FioFOpenFileSp(std::string_view filename, std::
 	if (subdir == Subdirectory::None) {
 		buf = filename;
 	} else {
-		buf = fmt::format("{}{}{}", _searchpaths[sp], _subdirs[subdir], filename);
+		buf = fmt::format("{}{}", FioGetDirectory(sp, subdir), filename);
 	}
 
 	auto f = FileHandle::Open(buf, mode);
@@ -871,6 +874,50 @@ extern void CocoaSetApplicationBundleDir();
 #else
 	_searchpaths[Searchpath::ApplicationBundleDir].clear();
 #endif
+
+	/* Look for Atari release of Transport Tycoon Deluxe for original data files */
+	std::string config_file_path;
+	const std::string atari_ini_filename = "Atari/Transport Tycoon Deluxe/installpath.ini";
+
+	_searchpaths[Searchpath::TransportTycoonDeluxeDir].clear();
+
+#ifdef WITH_COCOA
+extern std::string CocoaGetAppSupportDir();
+	config_file_path = CocoaGetAppSupportDir();
+
+	if (!config_file_path.empty()) {
+		AppendPathSeparator(config_file_path);
+		config_file_path += atari_ini_filename;
+	}
+#else
+	config_file_path = GetHomeDir();
+
+	if (!config_file_path.empty()) {
+		AppendPathSeparator(config_file_path);
+		config_file_path += ".local/share/";
+		config_file_path += atari_ini_filename;
+	}
+#endif
+
+	if (!config_file_path.empty()) {
+		size_t installpath_len;
+		std::unique_ptr<char[]> installpath = ReadFileToMem(config_file_path, installpath_len, MAX_PATH);
+
+		if (installpath != nullptr && installpath_len > 0) {
+			std::string ttd_path = installpath.get();
+			AppendPathSeparator(ttd_path);
+
+#ifdef WITH_COCOA
+			/* The path provided is to the TTD.app/Contents/MacOS folder */
+			ttd_path += "../Resources/";
+#endif
+
+			ttd_path += "CD";
+			AppendPathSeparator(ttd_path);
+
+			if (FileExists(ttd_path)) _searchpaths[Searchpath::TransportTycoonDeluxeDir] = ttd_path;
+		}
+	}
 }
 #endif /* defined(_WIN32) */
 
diff --git a/src/fileio_type.h b/src/fileio_type.h
index 208dd43ce6..cef4a0d315 100644
--- a/src/fileio_type.h
+++ b/src/fileio_type.h
@@ -121,6 +121,7 @@ enum class Searchpath : uint8_t {
 	BinaryDir, ///< Search in the directory where the binary resides.
 	InstallationDir, ///< Search in the installation directory.
 	ApplicationBundleDir, ///< Search within the application bundle.
+	TransportTycoonDeluxeDir, ///< Search within the Transport Tycoon Deluxe data directory (if installed).
 	AutodownloadDir, ///< Search within the autodownload directory.
 	AutodownloadPersonalDir,///< Search within the autodownload directory located in the personal directory.
 	AutodownloadPersonalDirXdg, ///< Search within the autodownload directory located in the personal directory (XDG variant).
diff --git a/src/os/macosx/macos.mm b/src/os/macosx/macos.mm
index c8e134588a..493b0f119d 100644
--- a/src/os/macosx/macos.mm
+++ b/src/os/macosx/macos.mm
@@ -138,6 +138,17 @@ void CocoaSetApplicationBundleDir()
 	}
 }
 
+/**
+ * Returns the path to the user's Application Support folder.
+ */
+std::string CocoaGetAppSupportDir()
+{
+	NSArray *paths = NSSearchPathForDirectoriesInDomains(NSApplicationSupportDirectory, NSUserDomainMask, YES);
+	NSString *appSupportPath = [paths firstObject];
+
+	return [appSupportPath UTF8String];
+}
+
 /**
  * Setup autorelease for the application pool.
  *
diff --git a/src/os/windows/win32.cpp b/src/os/windows/win32.cpp
index 0d2891836e..0d2e45fc79 100644
--- a/src/os/windows/win32.cpp
+++ b/src/os/windows/win32.cpp
@@ -322,6 +322,25 @@ void DetermineBasePaths(std::string_view exe)
 
 	_searchpaths[Searchpath::InstallationDir].clear();
 	_searchpaths[Searchpath::ApplicationBundleDir].clear();
+	_searchpaths[Searchpath::TransportTycoonDeluxeDir].clear();
+
+	if (SUCCEEDED(SHGetFolderPath(nullptr, CSIDL_APPDATA, nullptr, SHGFP_TYPE_CURRENT, path))) {
+		std::string config_file_path(FS2OTTD(path));
+		AppendPathSeparator(config_file_path);
+		config_file_path += "Atari\\Transport Tycoon Deluxe\\installpath.ini";
+
+		size_t installpath_len;
+		std::unique_ptr<char[]> installpath = ReadFileToMem(config_file_path, installpath_len, MAX_PATH);
+
+		if (installpath != nullptr && installpath_len > 0) {
+			std::string ttd_path = installpath.get();
+			AppendPathSeparator(ttd_path);
+			ttd_path += "CD";
+			AppendPathSeparator(ttd_path);
+
+			if (FileExists(ttd_path)) _searchpaths[Searchpath::TransportTycoonDeluxeDir] = ttd_path;
+		}
+	}
 }