A hard disk platter is a component of a hard disk drive. All hard drives contain one or more hard disk platters, which are used to actually hold the data. They are composed of two main substances: a substrate material, which forms the bulk of the platter and gives it structure and rigidity, and a magnetic media coating which actually holds the magnetic impulses that represent the data. Hard disks get their name from the rigidity of the platters used, as compared to floppy disks and other media which use flexible platters, which aren't actually even considered platters.

The platters are where the data is recorded. For this reason the quality of the platters and particularly, their media coating, is critical. The surfaces of each platter are precision machined and treated to remove any imperfections, and the hard disk itself is assembled in a clean room to reduce the chances of any dirt or contamination getting onto the platters.