IMAS-Core is part of the Integrated Modelling and Analysis Suite at ITER. It provides a low-level library to read and write MDSplus (Introduction) and HDF5 (HDF5) formats. It also supports reading and writing FlexBuffers. For debugging purposes, it allows writing ASCII files. Notably, for keeping data in-memory when exchanging data between physics codes, it supports writing into memory. The IMAS-Core library includes Python bindings, making it useful for working with Python. It can be installed on different operating systems, including macOS and Windows. IMAS-Core is a versatile library that helps users write fusion data in a standard format used by various users and facilitates easy data exchange. Most of the code is written in C++, with wrappers on existing libraries for HDF5, MDSplus, and FlexBuffers.