The 'QuantNBody' package is a Python toolkit for quantum chemists/physicists interested in methods development to study quantum many-body problems ranging from electronic structure to condensed matter theory. It provides a quick and easy way to build matrix representations of bosonic and fermionic quantum many-body operators (e.g., Hamiltonians, spin or excitation operators) and get access to quantities/objects of interest (e.g., energies, reduced density matrices, many-body wave functions). The code includes various native functions and it is flexible enough to help users in building their own numerical tools to implement new methods.