This information can be gained from the Player Log file!

To retrieve your multiplayer games world seed:

Start up Valheim
From the main menu, click the button labelled Show Player.log hidden away at the bottom left corner of the screen. This will open a folder in Windows Explorer; minimize that and come back to Valheim for now, we'll double back to this in a minute.
Join your multiplayer server as normal. (Start Game -> Start->Join Game, yadda yadda. You know this.)
Once you've loaded into the game world that you want the world seed for, it's time to Alt+Tab back to the Valheim AppData folder that was opened up by that player.log button.
We want to open the text file named Player.log, or, depending on your windows settings, it may show up as simply Player.
In that document, you'll want to Ctrl + F or scroll down and look for the string Initializing world generator seed:. Your seed will appear right after that*. Remember, world seeds are case sensitive!

Originally posted by burn:

This information can be gained from the Player Log file!

To retrieve your multiplayer games world seed:

Start up Valheim
From the main menu, click the button labelled Show Player.log hidden away at the bottom left corner of the screen. This will open a folder in Windows Explorer; minimize that and come back to Valheim for now, we'll double back to this in a minute.
Join your multiplayer server as normal. (Start Game -> Start->Join Game, yadda yadda. You know this.)
Once you've loaded into the game world that you want the world seed for, it's time to Alt+Tab back to the Valheim AppData folder that was opened up by that player.log button.
We want to open the text file named Player.log, or, depending on your windows settings, it may show up as simply Player.
In that document, you'll want to Ctrl + F or scroll down and look for the string Initializing world generator seed:. Your seed will appear right after that*. Remember, world seeds are case sensitive!

Tried that one, there was no such thing i could find in the log

https://www.survivalservers.com/forum/index.php?/topic/10856-finding-server-seed/

cheers ill try that one

no idea who is your host... but can't you simply download world from server, enter it into singleplayer and see the seed in menu?

not sure if F2 displays seed *didn't try*

You can't. These instructions are for single player worlds and games hosted by a single player instance. Real dedicated servers have the seed hashed. The information is not available in this log file or any other I have found.

This is incorrect. The instructions linked to by @burn are accurate - the seed is stored as ASCII characters inside the binary file vhserver.fwl and you can get it with the strings or xxd commands on linux, or whatever hex editor you prefer on Windows. Source: I just pulled mine from my dedicated multiplayer server.