SharePoint Online: Upload a Folder using PowerShell

Requirement: Upload a Folder to SharePoint Online using PowerShell.

How to Upload a Folder to SharePoint Online Document Library?
You can upload a folder along with its contents to SharePoint. Here is how to upload a folder with its sub-folders and files to a SharePoint Online document library:

• Navigate to your SharePoint Online library >> Click on "Upload" from the toolbar, choose the "Folder" option. You'll get a Browse dialog box to select a folder from your computer.
  
• Select a folder to copy that entire folder to SharePoint Online library.

This will upload the folder and its contents from the local computer to SharePoint Online. Please note, this option doesn't work in IE 11. (But works in Edge, Google Chrome, and Firefox browsers!). You can also upload a folder with files and sub-folders to SharePoint Online modern libraries, just by Drag and Drop!

PowerShell to Upload a Folder to SharePoint Online (Including Folder, Sub-Folders, and Files):
Let's upload a folder to SharePoint Online using PowerShell. Here is my source:

Prerequisites: Make sure you have installed SharePoint Online Client Components SDK in your machine to use client-side assemblies. You can download it from https://www.microsoft.com/en-us/download/details.aspx?id=42038

Here is how to Upload a Folder to SharePoint Online Document Library using PowerShell:

#Load SharePoint CSOM Assemblies
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"

#Function to Check if Folder Exists. If not, Create the Folder
Function Ensure-SPOFolder()
{
    param
    (
        [Parameter(Mandatory=$true)] [string] $FolderRelativeURL
    )

    #Check Folder Exists
    Try { 
        $Folder = $Web.GetFolderByServerRelativeUrl($FolderRelativeURL)
        $Ctx.Load($Folder)
        $Ctx.ExecuteQuery() 
 
        #Write-host -f Green "Folder Already Exists!"
    }
    Catch {
        #Create New Sub-Folder
        $Folder=$Web.Folders.Add($FolderRelativeURL)
        $Ctx.ExecuteQuery()
        Write-host -f Green "Created Folder at "$FolderRelativeURL
    }
}

#Function to Upload a File to a SharePoint Online
Function Upload-SPOFile()    
{
    param
    (
        [Parameter(Mandatory=$true)] [string] $SourceFilePath,
        [Parameter(Mandatory=$true)] [string] $TargetFileURL
    )
    
    #Get the file from disk
    $FileStream = ([System.IO.FileInfo] (Get-Item $SourceFilePath)).OpenRead()
    #Get File Name from source file path
    $SourceFileName = Split-path $SourceFilePath -leaf
   
    #Upload the File to SharePoint Library
    $FileCreationInfo = New-Object Microsoft.SharePoint.Client.FileCreationInformation
    $FileCreationInfo.Overwrite = $true
    $FileCreationInfo.ContentStream = $FileStream
    $FileCreationInfo.URL = $TargetFileURL
    $FileUploaded = $TargetFolder.Files.Add($FileCreationInfo)
  
    $Ctx.ExecuteQuery()  
    #Close file stream
    $FileStream.Close()
    Write-host "File '$TargetFileURL' Uploaded Successfully!" -ForegroundColor Green
}
 
#Main Function to upload a Local Folder to SharePoint Online Document Library Folder
Function Upload-SPOFolder()
{ 
    param
    (
        [Parameter(Mandatory=$true)] [string] $SourceFolderPath,
        [Parameter(Mandatory=$true)] [Microsoft.SharePoint.Client.Folder] $TargetFolder        
    )

    #Get All Files and Sub-Folders from Source
    Get-ChildItem $SourceFolderPath -Recurse | ForEach-Object {
        If ($_.PSIsContainer -eq $True) 
        {
            $FolderRelativeURL = $TargetFolder.ServerRelativeURL+$_.FullName.Replace($SourceFolderPath,"").Replace("\","/")
            If($FolderRelativeURL) 
            {
                Write-host -f Yellow "Ensuring Folder '$FolderRelativeURL' Exists..."
                Ensure-SPOFolder -FolderRelativeURL $FolderRelativeURL
            }  
        }
        Else
        {
            $FolderRelativeUrl = $TargetFolder.ServerRelativeURL + $_.DirectoryName.Replace($SourceFolderPath,"").Replace("\","/")
            $FileRelativeURL = $FolderRelativeUrl+"/"+$_.Name
            Write-host -f Yellow "Uploading File '$_' to URL "$FileRelativeURL
            Upload-SPOFile -SourceFilePath $_.FullName -TargetFileURL $FileRelativeURL
        }
    }
}

#Set parameter values
$SiteURL="https://crescent.sharepoint.com/sites/marketing"
$LibraryName="Documents"
$SourceFolderPath="C:\Users\salaudeen\Desktop\Marketing\Documents"
 
#Setup Credentials to connect
$Cred= Get-Credential
$Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Cred.Username, $Cred.Password)
 
#Setup the context
$Ctx = New-Object Microsoft.SharePoint.Client.ClientContext($SiteURL)
$Ctx.Credentials = $Credentials
      
#Get the Target Folder to Upload
$Web = $Ctx.Web
$Ctx.Load($Web)
$List = $Web.Lists.GetByTitle($LibraryName)
$Ctx.Load($List)
$Ctx.Load($List.RootFolder)
$Ctx.ExecuteQuery() 
 
#Call the function to Upload All files & folders from local folder to SharePoint Online
Upload-SPOFolder -SourceFolderPath $SourceFolderPath -TargetFolder $List.RootFolder

This PowerShell uploads the given folder, sub-folder, and files to SharePoint Online.

Here is the PnP PowerShell version of this script which uploads a folder with its subfolders and files: Upload Multiple files to SharePoint Online using PnP PowerShell