A 60-year-old man presented with a history of an acute episode of mono-ocular involvement and several acute spinal cord episodes from 1988 to 1991. Multiple MRIs of the spinal cord and brain and cerebrospinal fluid analysis were consistent with a clinical diagnosis of multiple sclerosis (MS). Following this, there was a quiescent period of four to five years, after which he reported progressive weakness and spasticity of lower limbs with urgency and precipitancy of urine. He was put on a ketogenic diet (KD) as a monotherapy in 2016. Within one month of starting the KD, his balance and weakness improved, and there was good bladder control. He continued KD for 18 months, after which he followed it inconsistently and eventually stopped KD, going back to his original diet. His weakness increased gradually until he was wheelchair-bound, and his precipitancy greatly worsened. He was put back on KD and has improved again to the extent that his stamina has increased, he can walk with the help of a cane, and his continence is good. Dietary therapy has a large role in the management of secondary progressive multiple sclerosis (SPMS) and, as in this case, may be effective even as a single-mode therapy. This is probably the first reported case of improvement in SPMS using KD as a monotherapy.