proEQUIB performs plasma diagnostics and abundance analysis of ionized gaseous nebulae from measured emission-line fluxes. Written in IDL/GDL, the library determines interstellar extinction, dereddens observed fluxes, and derives electron temperatures, electron densities, ionic abundances, atomic level populations, line emissivities, and critical densities. The package includes routines for collisionally excited lines based on the statistical-equilibrium approach for multi-level atoms of the Fortran program EQUIB (ascl:1603.005), and recombination-line routines for selected heavy-element ions. proEQUIB uses AtomNeb to read atomic data such as collision strengths, transition probabilities, and recombination coefficients, and is suitable for both integrated spectra and spatially resolved integral-field spectroscopy studies.