An IOU is a promise of money, goods, services, or other items of value, and may be either written or verbal. An IOU is less formal than a contract such as a promissory note, and, unless in the form of a written document stating that the value discussed will be repaid within a certain time, is generally not held to be legally binding.

While it appears as an acronym, it is a homophone for I owe you.

IOU is also a cellular phone service on Vodafone New Zealand where if a customer sends the letters IOU by form of SMS (text messaging) to 464, he/she will get a NZ$2.00 top-up, which is repaid when the "debtor" next tops up his/her phone.

IOU can also refer to an investor-owned utility (energy) company.