Samsung Electronics GmbH
Hello,
Thank you for your enquiry.
The Samsung SSD 870 EVO SATA III 2.5 inch - 1 TB MZ-77E1T0B/EU is compatible with the iMac (Retina 4K, 21.5″, 2017).
The SSD has the following connections: SATA 6 Gb/s interface, compatible with SATA 3 Gb/s & SATA 1.5 Gb/s interface.
If in doubt, please contact Apple customer support.
Best regards from Eschborn
Your Samsung Service Team

The Samsung 870 EVO SSD (1000 GB, 2.5") is not directly compatible with the iMac end of 2017 (21.5 inch, 4K), as this iMac does not have internal SATA III ports for 2.5-inch SSDs that could be easily replaced.

To use this SSD in your iMac, you would need an external solution, such as a USB-C adapter or an external SSD enclosure that connects the SATA III SSD to the iMac via USB-C or Thunderbolt 3.