Which SteamOS-like Linux distro can I install alongside my existing system?

Bazzite is probably the closest match if you want a SteamOS-style gaming setup. It’s based on Fedora Atomic and includes gaming-focused tools, while still being installable alongside another operating system if you carefully choose the target partition. Make sure to back up your data and select manual or custom partitioning during setup.

CachyOS is worth considering if you want something Arch-based. SteamOS is also based on Arch, but CachyOS is a conventional desktop distribution rather than an identical SteamOS replacement. It gives you more flexibility and can be installed to a selected partition, though you’ll need to configure more things yourself.

There isn’t really one official SteamOS derivative. Many alternatives borrow the idea of a gaming-focused interface, but they’re based on different systems. Bazzite is Fedora-based, while CachyOS, EndeavourOS, and similar options are Arch-based. For the most Steam Deck-like experience, I’d start by investigating Bazzite; for a more traditional customizable desktop, try CachyOS.

If you mainly want Plasma and an Arch foundation, regular Arch with KDE Plasma—or a friendlier Arch-based installer—may be enough. Just keep in mind that these systems won’t behave exactly like SteamOS, especially regarding its immutable design and console-style management. Whichever option you choose, test it from a live USB first and verify the installer’s partition settings before proceeding.