Per diem (Latin for "per day" or "for each day") or daily allowance is a specific amount of money that an organization gives an individual, typically an employee, per day to cover living expenses when travelling on the employer's business.

A per diem payment can cover part or all of the expenses incurred. For example, it may include an accommodation allowance or it may only cover meals (with actual accommodation costs reimbursed separately or be prepaid). Travel, particularly by motor vehicles, is often reimbursed at a rate determined only by distance travelled, e.g., the US business mileage reimbursement rate.

Fixed per diem (and per mile) rates eliminate the need for employees to prepare, and employers to scrutinise, a detailed expense report with supporting receipts to document amounts spent while travelling on business. Instead, employers pay employees a standard daily rate without regard to actual expenditure. In some countries, the income tax code specifies a maximum daily allowance: although an employer may pay a higher rate, the excess is subject to income tax.