The .thz file format aims to provide the terahertz spectroscopy community with a simple but powerful format to archive, collaborate and exchange data in a standardised fashion. The .thz files use the HDF5 format (www.hdfgroup.org) that many tools, including Matlab and Python, can read natively. The file structure is designed to collate the time-domain waveforms of sample and reference measurements and their metadata, such as sample temperature, sample thickness and other helpful information.