Connect an external display to your iMac
The Thunderbolt 3 (USB-C) ports on your iMac support video output. You can use an external display, a projector, or an HDTV with your iMac.
To connect this device to a Thunderbolt 3 (USB-C) port
Use this cable or adapter
Thunderbolt 3 (USB-C) display
Thunderbolt 3 (USB-C) cable
USB-C (USB 3.1) cable
Thunderbolt 2 display
Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter
HDMI display or HDTV
USB-C to HDMI adapter or a USB-C Digital AV Multiport adapter
VGA display or projector
USB-C VGA Multiport adapter
DisplayPort or Mini DisplayPort display
USB-C to DisplayPort or Mini DisplayPort adapter
DVI display or projector
USB-C to DVI adapter
If you have an HDTV connected to an Apple TV, you can use AirPlay to mirror your iMac screen in up to 1080p HD. See Use AirPlay on your Mac for details.
Adapters and other accessories are sold separately. Visit apple.com, your local Apple Store, or other resellers for more information and availability. Review the documentation or check with the manufacturer to make sure you choose the right product. See Adapters for the Thunderbolt 3 or USB-C port on your Mac or iPad Pro.
Learn more. See the Apple Support article Use external monitors with your Mac for details about about extending the desktop, video mirroring, and other display preferences. To troubleshoot an issue with an external display, see Get help with video issues on external displays connected to your Mac.