The Earth Science Box Modeling Toolkit (ESBMTK) is a Python library designed for building and analyzing box models in Earth science. It uses a modular, object-oriented approach to study topics like the long-term carbon cycle and the impact of atmospheric CO₂ changes on seawater chemistry. ESBMTK allows users to define models in a straightforward and readable way, which also serves as documentation. These model definitions are then converted into equations and solved using standard numerical libraries. The toolkit includes features for common box modeling tasks such as gas exchange between the ocean and atmosphere, marine carbonate chemistry, and isotope calculations. ESBMTK has been effectively used in both teaching and research settings. While the library is continually being improved, its core interface is stable and comes with extensive documentation.