Background: Several reports on neurological complications associated with SARS-CoV-2 infection have been published. However, systematic description on intensive care unit acquired weakness (ICUAW) are still missing. Methods: The objective was to determine the incidence and characteristics of critical illness polyneuropathy (CIN) and myopathy (CIM) in patients with severe COVID-19. We also aimed to describe the electrophysiological features and their relation to plasma biomarkers for neuronal injury. This was a prospective observational intensive care unit cohort study. All adult patients admitted to the general intensive care unit (ICU) at Uppsala University Hospital, Uppsala, Sweden, between March 13 and June 8, 2020 were screened for inclusion. Patients with PCR confirmed COVID-19 were included. All patients were admitted to intensive care treatment due to severe COVID-19, including intravenous anaesthesia, opioid anaelgesia, neuromuscular blockade and mechanical ventilation. Associations of clinical, electrophysiological (sensory and motor conduction studies and electromyography) and biomarker data [neurofilament light chain (NfL), glial fibrillary acidic protein (GFAp) and tau] were studied between COVID-19 patients who developed CIN/CIM and those who did not. Results: 111 COVID-19 patients were included, 11 (11 males, mean age: 64 years) developed CIN/CIM whereas 100 (74 males, mean age: 61 years) did not (non-CIN/CIM). The CIN/CIM incidence was higher in COVID-19 patients compared to a general ICU-population treated during 2019 (9.9% vs 3.4%). In particular CIN was more frequent in the COVID-19 ICU cohort (50%) compared with the non-COVID-19 ICU cohort (0%, p=0.008). NfL and GFAp levels were higher in the CIN/CIM group both at the early (<9 days) and late time points (>11 days) compared with the non-CIN/CIM group (both p=0.001) and correlated with nerve amplitudes. Conclusions: CIN/CIM, in particular CIN, were more prevalent among COVID-19 patients than an ICU treated control cohort and should be considered in the differential diagnostic workup and the further rehabilitation of COVID-19 patients. COVID-19 patients who later developed ICUAW had significantly higher NfL and GFAp in the early phase of ICU care, which suggests their potential as predictive biomarkers. Trial registration: The study protocol was registered (ClinicalTrialsID:NCT04316884). Mechanisms for Organ Dysfunction in Covid-19 (UMODCOVID19) March 18, 2020.