GCMS-ID (Gas Chromatography Mass Spectrometry compound IDentifier) is a webserver (https://www.gcms-id.ca) designed to help researchers identify compounds, annotate GC-MS spectra and label GC chromatograms from GC-MS experiments. GCMS-ID would be particularly useful for those working in the fields of metabolomics, exposomics, drug testing, natural product characterization and environmental chemistry. GC-MS experiments produce both electron impact mass spectra (EI-MS) and retention index (RI) data. Matching measured EI-MS, RI or EI-MS + RI data to known (or predicted) reference RI, EI-MS or RI+EI-MS data enables compound identification in GC-MS studies GCMS-ID allows users to submit either a compound structure (as a SMILES string or as a drawn structure) or experimental GC-MS data (RI, EI-MS and/or RI+EI-MS).