SharePoint Online: How to Make a List Read-Only?

Requirement: Make a SharePoint Online list to read only.

How to make a list read only in SharePoint Online?

Well, there is no direct way to set a SharePoint Online list to read-only. However, we can follow this workaround to make a list to read-only mode.

• Navigate to your SharePoint Online List >> Click on Settings gear and choose "List Settings"
• Click on "Permissions for this list" link under "Permissions and Management" group of the list settings page.
• Click on "Stop Inheriting Permissions" button in the ribbon and confirm the prompt, if the list is not with broken permissions already.
  
• Select users and groups that has permissions other than read. E.g. Full Control, Edit, contribute, Etc. and click on "Edit User Permissions" button in the ribbon.
  
• In the Edit Permissions page, Select "Read" permissions and Click on "OK" to reset everyone's permission to "Read".
  

PowerShell to make a list read only in SharePoint Online

Lets automate above steps to make a SharePoint Online list or library to read only.

#Parameter
$SiteURL = "https://Crescent.sharepoint.com/sites/PMO"
$ListName= "Projects"
 
#Connect to PnP Online
Connect-PnPOnline -Url $SiteURL -UseWebLogin
 
#Get the Web and List
$Web = Get-PnPWeb
$List = Get-PnPList -Identity $ListName -Includes HasUniqueRoleAssignments, RoleAssignments

#Break Permissions of the List
If ($List.HasUniqueRoleAssignments -eq $False)
{
    Set-PnPList -Identity $ListName -BreakRoleInheritance -CopyRoleAssignments
}
 
#Get Read Permission Level
$ReadPermission = Get-PnPRoleDefinition -Identity "Read"

#Grant "Read" permissions, if its not granted already
$List.RoleAssignments | ForEach-Object {
    #Get the user or group of the assignment - Handle error for orphans
    $Member = Get-PnPProperty -ClientObject $_ -Property Member -ErrorAction SilentlyContinue

    If($Member.IsHiddenInUI -eq $False)
    {
        Get-PnPProperty -ClientObject $_ -Property RoleDefinitionBindings | Out-Null
 
        #Check if the current assignment has any permission other than Read or related
        $PermissionsToReplace = $_.RoleDefinitionBindings | Where {$_.Hidden -eq $False -And $_.Name -Notin ("Read", "Restricted Read", "Restricted Interfaces for Translation")}
        
        #Grant "Read" permissions, if its not granted already
        If($PermissionsToReplace -ne $Null)
        {
            $_.RoleDefinitionBindings.Add($ReadPermission)
            $_.Update()
            Invoke-PnPQuery
            Write-host "Added 'Read' Permissions to '$($Member.Title)'" -ForegroundColor Cyan
        }
    }
}
#Reload List permissions
$List = Get-PnPList -Identity $ListName -Includes RoleAssignments

#Remove All permissions other than Read or Similar
$List.RoleAssignments | ForEach-Object {
    #Get the user or group of the assignment - Handle error for orphans
    $Member = Get-PnPProperty -ClientObject $_ -Property Member #-ErrorAction SilentlyContinue | Out-Null    
    If($Member.IsHiddenInUI -eq $False)
    {
        Get-PnPProperty -ClientObject $_ -Property RoleDefinitionBindings | Out-Null
 
        $PermissionsToRemove = $_.RoleDefinitionBindings | Where {$_.Hidden -eq $False -And $_.Name -Notin ("Read", "Restricted Read", "Restricted Interfaces for Translation")}
        If($PermissionsToRemove -ne $null)
        {
            ForEach($RoleDefBinding in $PermissionsToRemove)
            {
                $_.RoleDefinitionBindings.Remove($RoleDefBinding)
                Write-host "Removed '$($RoleDefBinding.Name)' Permissions from '$($Member.Title)'" -ForegroundColor Yellow    
            }
            $_.Update()
            Invoke-PnPQuery
        }
    }
}
Write-host "List is set to Read-Only Successfully!" -f Green

This PowerShell script resets all permissions with "Read". Please note, These methods doesn't control Site Collection Administrators!