Yes, and you never have to turn it on. NymVPN's kill switch is built in and always active on every platform: Windows, macOS, Linux, Android, and iOS. If your VPN connection drops, traffic is blocked rather than allowed to leak through your regular connection.
What a kill switch does
Without a kill switch, a dropped VPN connection fails open: your device quietly falls back to your normal internet connection, exposing your real IP address and traffic until you notice. A kill switch makes the connection fail closed instead: no tunnel, no traffic.
No toggle, by design
In many VPN apps the kill switch is a setting you have to find and enable. In NymVPN it is part of the tunnel design itself. There is no toggle to forget, and no way for an app crash or network change to silently expose you. Protection that depends on a checkbox is protection that fails when you forget the checkbox.
Extra protection on Android
Android adds two system-level options worth knowing: Always-on VPN, which starts NymVPN with your device and survives reboots, and Block connections without VPN for the strictest setup. See Does NymVPN offer Always-on/Kill switch features on Android?
Local devices still work
The kill switch blocks traffic from leaving your device outside the tunnel; it doesn't have to cut you off from your own network. If you need to reach a printer, Chromecast, or other local device while connected, enable LAN bypass. See What is the LAN bypass option used for?
To verify your connection isn't leaking, see How can I verify the security of my VPN connection?