A checkwriter is a device for protecting a check by preventing the monetary amount for which it was written from being raised or altered.

The person preparing a check positions the check in the checkwriter so its print-heads are centered over the field on the check where the amount of the check would otherwise be written out in words. Using a series of levers or buttons on the checkwriter's control panel, the operator enters the monetary amount of the check. This amount is then printed onto the check by the operator pulling a lever on the side of the unit (or by pressing a button on electric units). This brings the print-heads down upon a wide inked (usually multicolor) ribbon through which they print the selected amount on the check. Entering $6,762.64, for example, will produce text reading "TheSum6762dol's64cts".

There is a series of (usually) horizontal indentations on a pressure bar brought up underneath the check during this process, which matches similar indentations on the print-heads. This embosses the numeric amount through the paper of the check form, strongly discouraging any attempt to alter or raise this amount.

Any person, business, or organization which regularly prepares remittances by issuing high-amount checks is strongly advised to obtain and use a checkwriter. Financial institutions regularly use them to prepare cashier's checks, and they are also used by issuers of money orders such as the United States Postal Service.

The Paymaster® Series 8000 is a good example of a widely supported manual checkwriter. Other models and technologies also exist, such as units incorporating a dot-matrix printer, or which also perform other functions such as affixing a datestamp or automatic signature, or maintaining an account balance.