Is a Sleeper league ID secret?

No. It identifies the league but grants nothing on its own — anyone with the ID can read public league data through Sleeper's API, and nobody can change your league with it.

Where is the league ID in the Sleeper app?

It is not displayed. Open League Settings, tap Share League, copy the link, and read the long number out of the link.

Why is my league ID different from last year?

Sleeper creates a new league object each season and links it to the previous one. The ID changes every year; the chain preserves the connection.

How to find your Sleeper league ID

Your Sleeper league ID is the long number in the league URL. Here is where to find it on desktop and in the mobile app, and what to do when you only have the app.

A Sleeper league ID is an 18-digit number that identifies one league for one season. Almost every tool that reads your league — including this one — asks for it, and Sleeper never labels it as an ID anywhere in the interface, which is why it is hard to find.

On the web

Open sleeper.com and sign in.
Open the league you want.
Look at the address bar. The URL looks like sleeper.com/leagues/1388555898586136576/team.
The long number between /leagues/ and the next slash is your league ID.

https://sleeper.com/leagues/1388555898586136576/team
                        └──────────────────┘
                          your league ID

In the mobile app

The app never shows the ID directly, so you have to get a link out of it:

Open the league, tap the gear icon for League Settings.
Tap Share League, then copy the link.
Paste it anywhere you can read text. The long number in the link is your league ID.

If you would rather not hunt for it at all, you can enter your Sleeper username instead and pick the league from a list.

Why the ID changes every year

This catches people out. Sleeper does not reuse a league ID across seasons — each year your league is created as a new object with a new ID, linked to the previous season by a field called previous_league_id. So the ID you copied last season is not the one in your address bar today.

That matters when a tool asks for an ID: give it the ID of an old season and it can only see that season and earlier, because the chain is walked backwards. If you want your whole history, use the current season's ID.