Giffgaff is a mobile phone service based in the United Kingdom. It operates as a mobile virtual network operator using the O2 network. It is wholly owned by O2 and launched on 25 November 2009.

Giffgaff differs from conventional mobile phone operators in that the users of the service may also participate in certain aspects of the company's operation, e.g. sales, customer service and marketing. In return for this activity, the user receives remuneration through a system called "Payback".

Giffgaff was founded by Gav Thompson, the head of brand strategy at O2, who is reported to have come up with the idea after studying Web 2.0 businesses in San Francisco in 2008. The principles behind the formation of the company were set out in the Giffgaff manifesto.

The word giffgaff is purported to be Scottish English meaning mutual giving and was chosen to fit with the principles outlined in the manifesto. The strapline for Giffgaff was "the mobile network run by you" to reflect the fact that some users of the service help run various aspects of the operation.