The present disclosure relates to a mobile device grip assistive device that can improve convenience and economics in gripping of a mobile device such as a smart phone, and provides a mobile device grip assistive device including disposed in an up-down direction behind a mobile device and having flexibility, and a mechanism portion having a hole through which the strap can be disposed, and having a holding space in which friction is provided by an elastic restoration force of the strap when a finger is positioned between the front surface of a plate member and the rear surface of the strap.