The COVID-19 pandemic has led to a radical lifestyle change, which may unintendedly change physical activity levels. We aimed to perform a systematic review to investigate the physical activity changes in people with neurological diseases, and to examine the relationship between physical activity and disease symptoms, and psychosocial factors. The review was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. A systematic search of the literature across five databases (PubMed, CINAHL, Web of Science, SCOPUS, and Cochrane Library) was carried out using the keywords relating to COVID-19, physical activity, sedentary behaviour, exercise, and the name of the neurological diseases. The systematic search was updated on 4 February 2021 with the same keywords. Fourteen studies (n = 7662 persons with neurological diseases, n = 1663 healthy controls) were eligible for this review. The study populations were Parkinson disease (n = 7), dementia (n = 1), multiple sclerosis (n = 1), spinal cord injury (n = 1), hereditary spastic paraplegia (n = 1), neuromuscular diseases (n = 1), Charcot-Marie-Tooth neuropathy (n = 1), and epilepsy (n = 1). Thirteen studies reported a decreased physical activity level, one study reported a high interruption rate of physiotherapy/rehabilitation. Furthermore, the physical activity reduction was associated with worse disease symptoms, depression, perceived health, and mental and physical components of quality of life. The COVID-19 pandemic has a negative impact on the physical activity levels of people with neurological diseases, and this change was related to the worsening of disease symptoms and psychosocial factors. Registration number A protocol of the review was registered with the PROSPERO database (CRD42020207676).