This SharePoint tutorial, we will discuss how to give unique permission to a folder in SharePoint Online document library.
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Here, I have a SharePoint document library which has few folders like below:

We wanted to give unique permission to the folders inside the document library in SharePoint Online. Here, we are using SharePoint Online document library modern experience.
How to give unique permission to folder in SharePoint Online
Now, follow the below steps to give unique permission to a folder in the SharePoint Online document library.
Open the document library and click on the ECB menu for the particular folder for which you want to provide the unique permission.
Then click on Manage access.

In the Manage Access page, you can see who all are having access to the folder in the SharePoint document library.

Now for every group, you can click on the arrow and click on Stop sharing for which you want to remove the access.

By default SharePoint site owners will have access to the folder in the SharePoint document library. We can remove it by clicking on the Advanced link.
By default I remove the SharePoint site members and the visitors group like below:

And it should looks like below, only Site Owners have permissions.

Now, we will see how to give permission to a particular user to the folder in the document library.
Click on the + icon and then provide the user name like below:

Click on Grant Access button, and then you can see the user is having access to the folder like below:

Now, you can see here, If I logged in with another user except Bhawana, then they can not see the Finance folder like below:

If I logged with Bhawana user, I can see the Finance folder like below:

How to give links in SharePoint Online Left Navigation
Now, we will see how to give links in left navigation in SharePoint Online.
I open the folder in the browser and copy in the whole URL which looks like below:
https://tsinfo.sharepoint.com/sites/SharePointSky/Company%20Documents/Forms/AllItems.aspx?viewid=285edb5a%2Da73d%2D400a%2Db018%2Ddadc517171ec&id=%2Fsites%2FSharePointSky%2FCompany%20Documents%2FFinance
In the left navigation, I added the path like below:

But the problem is, when I loged in with other users (who does not have the permission to the folder), still they can see the link in the SharePoint Online left navigation.

Then I have change the folder URL like below in the left navigation like below:
https://tsinfo.sharepoint.com/sites/SharePointSky/Company%20Documents/Finance/
And it looks like below:

After this, you can see the below diagram, only in Bhawana login I can see the folder in the left navigation in SharePoint. But in the other login, I can not see the SharePoint Online left navigation.

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In this SharePoint tutorial, we discussed how to give unique permission to folder in sharepoint online document library.
Also, we saw how to give the folder URL in the SharePoint Online left navigation.
I am Bijay a Microsoft MVP (10 times – My MVP Profile) in SharePoint and have more than 17 years of expertise in SharePoint Online Office 365, SharePoint subscription edition, and SharePoint 2019/2016/2013. Currently working in my own venture TSInfo Technologies a SharePoint development, consulting, and training company. I also run the popular SharePoint website EnjoySharePoint.com
Is it possible for a user to exclusively have an access control role for document library folders without being able to read/edit? The idea is to have one person delegated to assign users permissions to all the folders without necessary being able to see the folder contents themselves.