Will a USB-C Adaptor; fit into what Apple calls the USB 3 port on the new iMac 2021

Actually, Apple's own website refers to it as "USB 4" in conjunction with it being a Thunderbolt port. By the photo alone, one can visually identify them as USBc ports.

It's a little confusing, especially when "Thunderbolt" is mixed in. USB4/Thunderbolt is a protocol. USBc is a physical port.

USB4 is compatible and interchangeable with any prior flavors of USB, including USB3.1, USB2, and so on. The USB3 ports while slower than USB4, can still connect to slower USBA devices. It's just that those require require an adapter - as you noted - to plug a physical USBA device to the iMac's physical USBc port.

Do not entirely blame Apple for this one.

<rant>

The confusion stems from the USB Consortium (the council that defines and maintains the official USB specification). They have consistently messed up when it comes to clarity around USB-A, USB-B, USB-C, mini-USB, micro-USB (which are all physical port definitions) and USB1/USB2/USB3/USB3.1/USB3.2/USB4 (which are all protocol definitions).

They will say it's done in the name of backwards compatibility, but you almost need a Phd in astrophysics to understand the logic behind which physical/logical combinations actually work (not to mention the lack of clarity around cable standards (not all USB-C cables are created equally, but there is ZERO way to know this by looking at the cable).

In this case, ALL your ports are USB-C (the physical definition). SOME support USB-3(.1), SOME support USB4.

</rant>

When usb3 Thunderbolt3 and usb4 use same connector it would be as trouble some if it was the way you wanted it because a person who purchased a thunderbolt 3 device would think it would work because it was labeled usb-c rather than usb3 or 4

That adapter is a USBc -> USB-A. So yes, it plugs right into the USBc ports on the back of the new iMac and allow you to use old-generation USB-A devices.

yes

usb3 comes in both usb-a (the old one) and usb-c a connector it shares with thunderbolt 3

It will fit yes. Same port type. It will also fit in the identically shaped Thunderbolt / USB 4 ports but will of course only produce USB-3 speeds from it.

raspberryaddiction wrote:

Wish Apple would call it a USB C port instead of USB 3. That was my confusion.

To tell you what connector you need.

Thanks to all for this info! I've been wondering how anyone keeps it all straight. I'm printing the comments and saving them in my filing cabinet, for the rest of my Apple life.

I asked this exact question on a private board for higher-level members. 8^) So it is not just you.