CMasher is a Python library that provides 60+ scientific colormaps for Matplotlib, designed for perceptual uniformity and color-vision-deficiency accessibility. At runtime, importing cmasher registers every colormap into Matplotlib's colormap registry under a `cmr.` prefix, exposing forward, reversed, and for cyclic maps half-shifted variants. The package also ships a CLI (`cmr`), utility functions for combining, sub-selecting, sampling, viewing, and exporting colormaps, plus cross-application exporters for Tableau and R. The colormap definitions themselves are stored as static NumPy arrays, normalized/8-bit/hex text files, JSON `.jscm` files, and generated Python modules, making the project both a functional library and a curated data asset.