Provided are an optical sensor including graphene quantum dots and an image sensor including an optical sensing layer. The optical sensor may include a graphene quantum dot layer that includes a plurality of first graphene quantum dots bonded to a first functional group and a plurality of second graphene quantum dots bonded to a second functional group that is different from the first functional group. An absorption wavelength band of the optical sensor may be adjusted based on types of functional groups bonded to the respective graphene quantum dots and/or sizes of the graphene quantum dots.