DNS provider

The other variant needs a DNS record that points at the same server (or CDN) as the main address. Most often that is www:

If your provider does not allow a CNAME there, use an A record with the same IP address as the main domain (for example 203.0.113.10), plus an AAAA record if the main domain has one.

If the main address is www.example.com and the bare example.com has no record, add an A record for @ with the server's IP. A CNAME is not allowed on the bare domain unless your provider offers ALIAS, ANAME or CNAME flattening.

New records usually work within minutes; an existing record that you change can take as long as its TTL.

WordPress

WordPress already redirects to the host in Settings → General → Site Address (URL) (its canonical redirect), as long as the request for the other variant reaches the same WordPress installation.

1. Set WordPress Address (URL) and Site Address (URL) to the same main address, for example https://example.com.
2. Make sure the other variant has a DNS record and a certificate (see the DNS tab and your hosting tab). Without a certificate the browser stops before WordPress can redirect.
3. Clear the page cache.

If the other variant shows a different site or a hosting placeholder page, it is not pointed at your WordPress site. Fix that in the hosting panel.

cPanel

1. Certificate: in Security → SSL/TLS Status, check that both example.com and www.example.com are listed as covered. AutoSSL includes www when its DNS record points at the server; click Run AutoSSL after adding the record.
2. Redirect: add this at the top of the .htaccess in the site's root (File Manager, with Show Hidden Files on). It redirects www to the bare domain and http:// to https:// in one hop:

RewriteEngine On
RewriteCond %{HTTPS} off [OR]
RewriteCond %{HTTP_HOST} ^www\. [NC]
RewriteRule ^ https://example.com%{REQUEST_URI} [L,R=301]

For the opposite direction (bare domain to www), change the second condition to RewriteCond %{HTTP_HOST} !^www\. [NC] and the target to https://www.example.com%{REQUEST_URI}.

cPanel also has Domains → Redirects, but its www options are easy to set up as a loop. The .htaccess rule is more predictable.

Plesk

1. Certificate: in Websites & Domains → example.com → SSL/TLS Certificates, when you issue or renew the Let's Encrypt certificate, include the www subdomain.
2. Redirect: open Websites & Domains → example.com → Hosting & DNS → Hosting (Hosting Settings in older versions). Under Preferred domain, choose example.com or www.example.com. Plesk then sends a permanent 301 redirect from the other one.
3. Click OK.

nginx

In your site's file (/etc/nginx/sites-available/example.com on Debian and Ubuntu, /etc/nginx/conf.d/example.com.conf on Rocky, Alma and CentOS), give the other variant its own HTTPS server block that only redirects:

server {
    listen 443 ssl;
    listen [::]:443 ssl;
    server_name www.example.com;

    ssl_certificate     /etc/letsencrypt/live/example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;

    return 301 https://example.com$request_uri;
}

server {
    listen 80;
    listen [::]:80;
    server_name example.com www.example.com;

    return 301 https://example.com$request_uri;
}

The main server block keeps only server_name example.com;. The certificate must cover both names; with Certbot:

sudo certbot certonly --nginx -d example.com -d www.example.com
sudo nginx -t && sudo systemctl reload nginx

Apache and LiteSpeed

With server access (Apache). Add a virtual host for the other variant that only redirects (in /etc/apache2/sites-available/ on Debian and Ubuntu, /etc/httpd/conf.d/ on Rocky, Alma and CentOS):

<VirtualHost *:443>
    ServerName www.example.com
    SSLEngine on
    SSLCertificateFile    /etc/letsencrypt/live/example.com/fullchain.pem
    SSLCertificateKeyFile /etc/letsencrypt/live/example.com/privkey.pem
    Redirect permanent / https://example.com/
</VirtualHost>

Remove www.example.com from the ServerAlias of the main virtual host, get a certificate for both names (sudo certbot --apache -d example.com -d www.example.com) and reload:

sudo apachectl configtest && sudo systemctl reload apache2

(httpd instead of apache2 on Rocky, Alma and CentOS.)

Without server access (shared hosting, LiteSpeed). Use the .htaccess rule from the cPanel tab; it works on any host that reads .htaccess.

Cloudflare

1. In DNS → Records, make sure the other variant exists and is Proxied (orange cloud). Redirect rules only run on proxied traffic. Cloudflare's edge certificate covers example.com and www.example.com automatically.
2. Go to Rules → Overview, click Create rule and choose Redirect Rule (on some accounts: Rules → Redirect Rules). Start from the template Redirect from WWW to root, or set it up yourself:
   • When incoming requests match: Hostname equals www.example.com
   • Then: Dynamic, expression concat("https://example.com", http.request.uri.path)
   • Status code: 301, and turn on Preserve query string
3. Deploy.

Shopify

Shopify connects both example.com and www.example.com and redirects all of them to the primary domain. In Settings → Domains, check that both are listed as connected and that the one you want is the primary domain (Change primary domain if not). Shopify issues the certificates once the DNS records point at Shopify.