Abstract Assessing disease progression and informing clinical trials in peripheral neuropathy would benefit from objective and responsive fluid biomarkers closely linked to disease biology. This is particularly important in chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and Guillain–Barré syndrome (GBS), the most common inflammatory neuropathies, where reliable biomarkers of peripheral demyelination would help identify, and potentially measure, active disease and responses to treatment. We postulated that periaxin, a protein exclusively expressed by myelinating Schwann cells, could serve as a fluid biomarker of demyelinating peripheral neuropathy. We developed a single molecule array (Simoa)-based immunoassay to measure plasma periaxin in patients with CIDP (n = 45, including longitudinal samples across a discovery cohort and a validation cohort, for a total of 77 time points), GBS (n = 30, 66 time points), Charcot–Marie–Tooth disease (CMT, n = 20), CNS disease controls with multiple sclerosis (n = 30) and healthy controls (n = 30). We also evaluated whether periaxin is released in myelinating co-cultures following immune-mediated demyelination and axonal damage, comparing results with uninjured cultures. Plasma periaxin effectively distinguishes peripheral from CNS diseases, with significantly elevated levels in CIDP, GBS and CMT, but not in CNS disease or healthy controls (all P < 0.01). In CIDP, periaxin discriminates patients with active disease from those with inactive disease (P < 0.0001), and plasma levels decrease following treatment with intravenous immunoglobulin (IVIg). Elevated periaxin strongly predicts clinical worsening at 1 year [sensitivity 99%, specificity 72%, area under the curve (AUC) 0.86 (95% confidence interval, CI: 0.67–1)]. In GBS, peak levels of plasma periaxin and the ratio of periaxin to axonal biomarkers [neurofilament light chain (NfL) and peripherin] discriminate most cases of acute inflammatory demyelinating polyradiculoneuropathy (AIDP) from acute motor axonal neuropathy (AMAN), as classified by electrophysiology (sensitivity 100%, specificity 86%, AUC = 0.94, 95% CI: 0.81–1). Serial measurements showed that plasma periaxin levels peak 2 to 3 weeks after GBS symptom onset, followed by a gradual decline in the weeks thereafter. In vitro, periaxin is higher following immune-mediated demyelination compared with axonal damage and control conditions. Plasma periaxin is a biomarker of peripheral nerve demyelination. Combined with axonal fluid biomarkers and existing clinical scales, periaxin has the potential to improve the clinical management of peripheral neuropathies, accelerating advances in care and experimental research.