Adaptive radiation therapy (ART) has been proposed as a method to account for changes in head and neck cancer and normal tissues to enhance the therapeutic ratios. Online magnetic resonance-guided radiotherapy (MRgRT) using hybrid MR-Linac systems is a novel innovative application in ART for head and neck cancer. The concept of MR-Linac systems is the ability to acquire MR images for ART and also online imaging during treatment delivery. Daily ART allows to improve the targeting accuracy while avoiding organs at risk for head and neck cancer. Although an increasing number of studies related to clinical application and technical aspect of MRgRT in head and neck cancer have been published, MRgRT for ART of head and neck cancer remains in its infancy. The purpose of this article is to summarize and discuss the rationale, clinical implementation, and prospect of this promising adaptive radiotherapy modality for treating head and neck cancer.