Multiple sclerosis (MS) is an immune-mediated disease of the central nervous system, which leads, in many cases, to irreversible disability. More than 15 disease-modifying treatments (DMTs) are available for the treatment of MS. Clinical activity or activity at magnetic resonance imaging (MRI) are now used to assess the efficacy of DMTs, but are negative prognostic factors per se. Therefore, a biomarker permitting us to identify patients who respond to treatment before they develop clinical/radiological signs of MS activity would be of high importance. The number of circulating CD56bright natural killer (NK) cells may be such a biomarker. CD56bright NK cells are a regulatory immune population belonging to the innate immune system. The number of CD56bright NK cells increases upon treatment with interferon-beta, alemtuzumab, dimethyl fumarate, after autologous hematopoietic stem cell transplantation, and is higher in those who respond to fingolimod. In some cases, an increased number of CD56bright NK cells is associated with an increase in their regulatory function. In the current review, we will evaluate the known effect on CD56bright NK cells of DMTs for MS, and will discuss their possible role as a biomarker for treatment response in MS.