IO games grew up: 3D multiplayer browser games

In April 2015 a 19-year-old Brazilian developer called Matheus Valadares posted a game on 4chan in which you were a coloured circle in a petri dish, eating smaller circles and running from bigger ones. It was called Agar.io, it ran in a browser, and it started a genre. A year later, in March 2016, Steven Howse released Slither.io, the snake game with hundreds of players to a server, and for a while it felt as if every new web game ended in “.io”.

Those early io games had a lot in common: instant play in a browser, no account, huge crowds in one arena, and simple, flat, top-down graphics. The genre’s popularity peaked in the early 2020s and then cooled, as many games moved to phones. But the idea underneath, open a link and you are in a live world with other people, did not go away. It grew a third dimension.

Here is a short history and our picks of the 3D multiplayer browser games that carry the io idea forward. Picks are our opinions; facts were checked in October 2026 and sources are at the end.

What makes an io game

• It runs in the browser. No install, no launcher.
• You are in within seconds. Usually no account, or an optional one.
• It is multiplayer, live. Everyone you see is another real player.
• The rules fit in a sentence. Eat, grow, survive; or shoot, score, respawn.

The name comes from the .io web address many of these games used. Not every game on this list has one, but every one keeps the spirit.

The classics, still running

Agar.io went viral through the free games site Miniclip, and its developer followed it with Diep.io. Slither.io launched in browsers and on iOS and Android within two days of each other. It is free, and was famously paid for by adverts after each death, removable for US$3.99. They are flat and simple, and still surprisingly tense: the moment you are big enough to matter is the moment everyone is hunting you.

The 3D generation

Krunker

The io idea as a first-person shooter. Krunker, started in 2018 by Swiss developer Sidney de Vries and bought by FRVR in 2022, is free, plays without installing anything and supports full modding. Its blocky look is deliberate: it keeps the game fast on ordinary computers. The press release announcing the sale claimed more than 200 million players.

Shell Shockers

You are an egg with a gun, and so is everybody else. Shell Shockers, by Blue Wizard Digital, is a free multiplayer first-person shooter in the browser, listed on Wikipedia’s own page of io games under its old name, Shellshock.io. It is the funniest game on this list.

Smash Karts

Karts, arenas, weapons from loot boxes. Smash Karts is a free 3D multiplayer kart battle game in the browser, with phone apps as well. It is noisy, quick to learn and one of the easiest games here to start in a group.

AJ Cities: the live estate

We make this one, so read it knowing that. AJ Cities is a free 3D open-world game for adults set on the edge of Ajegunle, Lagos, and its live mode takes the io idea somewhere new: not an arena, but a place.

Sign in with your email (you must be 18 or over), make your own character and you arrive outside the estate’s main gate with other real players. Each estate holds up to 60, and when one fills another opens. Pick a world that is always day, always night or on real Lagos time; everyone in the same world sees the same sky.

• Get past the gate. Ask the armed police to let you in. Most nights about four in five who ask get in free; optional in-game tickets open more of the estate for 24 hours.
• Hear the same thing. The estate has one radio station, and every radio plays the same song at the same moment for every player.
• Go somewhere. The club, Prizm, has a set every night from 9 pm; the bush has a football pitch, with the beach beyond.
• Step away. Pause and you stay in the world as “away” for up to ten minutes.

There is no score and nobody wins. It is closer to the original io feeling, a crowd of real people in one shared space, than to a shooter. How the live estate works has the details. It is 18+ because the game around it, AJ’s story, deals with crime, police corruption and fighting.

Playing 3D io games well

• Use a mouse for shooters. Krunker and Shell Shockers are much harder on a trackpad.
• Pick the nearest server. Lag matters more in 3D than it did for snakes.
• Turn the graphics down. A steady frame rate beats shadows every time.
• Play the official site. Copies of popular io games on other sites can carry extra adverts or worse. See are browser games safe?

Where io games go next

The jump from a flat circle to a 3D world in a tab took less than a decade. What has not changed is the promise: send someone a link, and a few seconds later you are both in the same place. The arenas got bigger, the graphics got real and the crowds got smaller and friendlier, but the link is still the whole invitation. For more, see our multiplayer browser games and browser games to play with friends.

What are io games?

Browser-based multiplayer games, often on a .io web address, that you can join in seconds. The genre took off with Agar.io in 2015 and Slither.io in 2016.

Are there 3D io games?

Yes. Krunker and Shell Shockers are free 3D first-person shooters in the browser, and Smash Karts is a free 3D kart battle game.

How many players can join an AJ Cities estate?

Up to 60 players share each live estate. When one fills up, another opens.