MCT8 deficiency is an X-linked recessive disorder. We report the case of a 2-year-old Japanese boy with MCT8 deficiency caused by a novel frameshift variant, NM_006517.5(SLC16A2_v001):c.966dup [p.(Ile323Hisfs*57)]. He presented no head control and spoke no meaningful words, indicating severe developmental delay. Although missense or in-frame mutations of SLC16A2 are usually related to milder phenotypes and later-onset pyramidal signs, loss-of-function mutations are expected to cause severe clinical symptoms.