You're floating in the middle of a literal alien ocean. Your Lifepod is smoking, the Aurora is a screaming metal carcass on the horizon, and every shadow beneath your flippers looks like it has teeth. Subnautica is, by design, a lonely experience. It’s a game about isolation. But honestly? Sometimes isolation is boring. You want to show your friend that massive base you built in the Mushroom Forest, or maybe you just need a second pair of hands to help carry Titanium while you're dodging a Reaper Leviathan.
There is a huge problem, though. The developers, Unknown Worlds, never actually built a multiplayer mode. They’ve been pretty vocal about it over the years, explaining that adding co-op would have required a total rewrite of the game engine's architecture. So, if you’re looking for a "Join Game" button in the main menu, you're going to be looking for a very long time. It doesn’t exist.
The Nitrox project: Your only real path
Because the devs didn’t do it, the community did. If you want to know how to play Subnautica multiplayer, you have to talk about Nitrox. This is an open-source mod that has been in development for years. It is a massive, complex piece of software that essentially "tricks" the game into syncing data between two or more players.
It’s not a simple "install and go" situation. You aren't just clicking a button in the Steam Workshop. In fact, Subnautica doesn't even have official Steam Workshop support for mods like this. You have to go to the Nitrox website or their GitHub repository to get the files.
Does it actually work?
Kinda. Look, I’m going to be real with you: Nitrox is a feat of engineering, but it’s buggy. Expecting a flawless, AAA-quality co-op experience will only lead to heartbreak. You’ll see "ghosting" where your friend appears to be swimming through a wall. Sometimes the Aurora won't explode at the same time for both of you. Occasionally, a Seamoth might just... vanish.
But for many, the trade-off is worth it. Being able to pilot a Cyclops with a dedicated navigator and a guy on fire-extinguisher duty changes the entire vibe of the game. It turns a survival horror game into a chaotic road trip.
Setting up the server (The technical bit)
You can't just host a game from inside the Subnautica client. You have to run a separate server application. Usually, this means one person in your group acts as the host. They download the Nitrox launcher, and within that launcher, they start a "Server."
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If you're playing with someone in the same house, it’s pretty easy. You use your local IP address. But if you’re trying to play with a friend across the country, you’re diving into the world of port forwarding. You’ll need to go into your router settings and open port 11000 (that’s the default, anyway). If that sounds like gibberish, you might want to use a tool like Hamachi or Radmin VPN to create a virtual local network. It’s a bit old-school, like we’re back in 2012 playing Minecraft, but it gets the job done without messing with your router’s firewall.
The Version Mismatch Trap
This is where most people fail. Subnautica gets updated. Nitrox gets updated. If they aren't synced, the mod will crash your game before you even see the title screen.
Currently, Nitrox usually works best with the "Legacy" version of Subnautica on Steam. When the "Living Large" update (2.0) dropped a while back, it broke almost every mod in existence because it changed the game’s code from the old framework to a newer version of Unity. To fix this, you often have to right-click Subnautica in your Steam library, go to "Properties," then "Betas," and select the "legacy" branch. This rolls your game back to an older, more stable version that the modders have already figured out.
What actually syncs and what doesn't?
Don't expect every single fish to be in the exact same spot for both players. The mod prioritizes "important" stuff.
- Base Building: This is the most stable part. If you build a room, your friend sees the room. If they deconstruct a ladder, you're going to fall. It’s mostly seamless.
- Inventories: Usually works fine, though sometimes items can "desync" if you both try to grab the same piece of Copper at the exact same millisecond.
- The Story: This is the rocky part. Subnautica’s story is triggered by specific events and radio messages. If player A triggers a radio message, player B might not hear it. It’s best to have one person be the "mission lead" to keep the story progression from getting tangled.
- Vehicles: Piloting the Cyclops together is the peak experience, but be careful. If the connection is laggy, the physics engine can get confused, leading to the Cyclops spinning wildly into the air like a very expensive, very metallic frisbee.
Why didn't they just add official multiplayer?
Charlie Cleveland, the co-founder of Unknown Worlds, has addressed this dozens of times. When they started Subnautica, they were a tiny team. They had to choose: make a great single-player game, or make a mediocre multiplayer game. They chose the former.
By the time the game became a massive hit, the "bones" of the game were already set. Adding multiplayer to a game not built for it is like trying to turn a bicycle into a school bus while you're already riding it down a hill. They did eventually add some "social" features in Subnautica: Below Zero, but even that remained a single-player journey. This is why the third Subnautica game (currently in development) is such a big deal—the devs have confirmed they are building it with multiplayer in mind from day one.
Survival tips for a modded ocean
If you're going to commit to this, follow these rules to save your sanity.
- Save often. Not just in-game. Back up your actual save folder. Nitrox saves are stored in the mod folder, not the usual Subnautica save location.
- Clear the cache. If the world starts looking weird or chunks aren't loading, you might need to go into the server folder and clear the cell cache. It’s a common fix for long-running servers.
- One at a time. Don't try to dock two vehicles at the exact same time. The game's animations weren't meant to handle multiple actors in those sequences.
- Check the Discord. The Nitrox community is surprisingly active. If you get a weird error code, someone in their Discord has probably already found the fix.
Actionable steps to get started
Stop searching for a magic "multiplayer patch" and just follow this sequence. First, ensure you and your friends are all on the same game version; using the Steam "Legacy" beta branch is the safest bet for mod compatibility. Download the latest Nitrox installer from the official site (nitrox.ru) and run it as an administrator.
One person needs to take the hit and be the server host. If you aren't comfortable with port forwarding, download Radmin VPN, create a private room, and have everyone join it. This simplifies the connection process to a simple copy-paste of an IP address. Once the server is running, launch the game through the Nitrox launcher—never through Steam directly. If you see the "Multiplayer" button on the main menu, you've made it. If not, check your file paths in the Nitrox settings. Stick to these steps, keep your expectations realistic regarding bugs, and you'll be building underwater palaces with your crew in no time.