Multisite setup help – plain domain/subsite always redirects to domain with subdir multisite

You’re close.

I added a few more places to capture more scenarios and used the HTTP_HOST variable in the rule. The $1 is not needed in the condition as that would match what’s captured in the above condition.

<IfModule mod_rewrite.c>
    RewriteEngine On
    Rewritecond %{HTTP_HOST} !^www\.(.*)$ [NC] 
    RewriteCond %{REQUEST_URI} ^/subsite/? [NC] 
    RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [L,R=301]
</IfModule>

I recommend Made With Love’s .htaccess check to verify your testing.