Yes, the 27-inch iMac 2020 can be a monitor.
What it shows over a cable.

What makes the 2020 iMac a good monitor?

The iMac (Retina 5K, 27-inch, 2020) is the last Intel iMac and the newest one Encore supports. It is also the only iMac with a 10Gb Ethernet option.

Source: Apple's tech specs for the 2020 iMac.

It also runs the newest macOS of any Intel iMac: macOS 26 Tahoe. Encore Receiver needs only High Sierra 10.13 or later, so any version works.

Does the 2020 iMac have Target Display Mode?

No. Apple's Target Display Mode list stops at the 21.5-inch Mid 2014 iMac. No Retina 5K iMac has it, so Command-F2 does nothing here.

Apple also requires the connecting Mac to be from 2019 or earlier. So with a Mac from 2020 or later, every route to this iMac goes through software.

What does Encore show on a 2020 iMac?

Encore sends Retina iMacs at up to 2560 x 1440 and scales the picture to the panel. On a 5120 x 2880 panel, that is exactly half the resolution in each direction.

Text is sharp and crisp, but not every pixel is driven directly. Over a cable, lossless tiles keep still text pixel-exact at the size Encore sends. Every frame is timed to the iMac's refresh.

We expect 60 fps on this model. That is an expectation, not yet measured on every unit. Encore has not published latency figures yet.

What is Ultra, and does it work on the 2020 iMac?

Ultra is a third picture choice next to Sharp and Smooth. It sends every frame as ProRes 4444, encoded by your Mac's hardware. It is never the default; you turn it on per iMac.

The gain is color detail. In our measurements, Ultra scored 51 dB on colored text, against 27 dB for H.264. It is sharper, not faster.

Encore shows the Ultra choice only when the iMac and the cable qualify. On the 2020 iMac, that means:

• A cable. Ultra never runs over Wi-Fi.
• Enough speed. The panel is larger than 1920 x 1200, so Gigabit Ethernet isn't enough. A 1440p stream in ProRes 4444 needs about 1064 Mbit/s at 60 fps. Use Thunderbolt.
• A fast decode. The iMac decodes ProRes on its own processor. Encore measures that speed when the iMac starts, and offers Ultra only if it keeps up.

The stream is the panel size up to 2560 x 1440, never more. If decoding falls behind during a session, Encore switches back to the normal picture by itself.

Thunderbolt 3 or 10Gb Ethernet?

For most people, one Thunderbolt 3 or 4 cable with USB-C on both ends is the answer. A Mac with USB-C needs no adapter. Look for the lightning-bolt logo, because a plain USB-C charging cable won't do.

Gigabit Ethernet also works, at up to 1000 Mbit/s. That gives the sharp, lossless picture, but not Ultra at this panel size.

The 10Gb Ethernet option is a special case. Apple says that port supports 2.5Gb, 5Gb and 10Gb speeds. Encore's rule allows Ultra over Ethernet at 2.5 Gbit/s or more. So on paper, a 10Gb iMac wired to a Mac with fast Ethernet qualifies. We have not published measurements for that setup yet.

The cable guide lists parts, prices and traps for every iMac, including routers with slow ports.

Can the 2020 iMac show native 5K from another Mac?

Not with Encore. If every 5K pixel matters, there are other routes, each with its own catch.

• TargetBridge is free and open source. Its receiver needs macOS 14 on an Intel iMac, so a 2020 iMac qualifies. It streams over Thunderbolt Bridge, and marks 5K at 60 fps as experimental. Encore vs TargetBridge has the details.
• Paid apps such as RetinaRelay and Luna Display state native 5K, on their own setups.
• A driver board gives 5120 x 2880 at 60 Hz, but you remove the computer inside the iMac.

Use a 5K iMac as a monitor compares all of them for every 27-inch Retina model, with prices and sources.

Does AirPlay to Mac work on the 2020 iMac?

Yes, on macOS 12 Monterey or later. It needs no app and no cable. It is fine for mirroring or a quick extra screen, but less responsive than a cable for everyday work.

How to set up a 2020 iMac as a monitor

1. Install both apps.Encore on your Mac (macOS 14 or later) and Encore Receiver on the iMac.
2. Plug in one Thunderbolt cable.Use a Thunderbolt port on the back of the iMac. In Encore, the iMac's card offers Cable and marks it Recommended once it finds it.
3. Pair once.Type the 4-digit code from the iMac's screen into Encore on your Mac. After that, it connects by itself.
4. Pick the picture.Choose Sharp or Smooth on the iMac's card, or Ultra if it appears.
5. Arrange the display.The iMac shows up in System Settings, Displays. Drag windows onto it.

What else the 2020 iMac does with Encore

• Keyboard and mouse: over a cable, the iMac's keyboard and mouse can control your Mac. Press Control-Option-Command-Esc on the iMac to switch back.
• Speakers: your Mac's sound can play on the iMac's speakers.
• Camera and microphone: using the iMac's 1080p camera and its microphone from your Mac is coming soon, over a cable.
• StandBy: when it isn't your monitor, the iMac becomes a clock with the weather and your calendar.

One caveat is power. Apple's power figures list 74 W at idle for this model. Let it sleep, or use StandBy, when it isn't your monitor.

The 2020 iMac page has both downloads and the cable for this model. Not sure which iMac you have? Check the compatibility page.