BoltzTraP2 is a software package for calculating a smoothed Fourier expression of periodic functions and the Onsager transport coefficients for extended systems using the linearized Boltzmann transport equation. It uses only the band and k -dependent quasi-particle energies, as well as the intra-band optical matrix elements and scattering rates, as input. The code can be used via a command-line interface and/or as a Python module. It is tested and illustrated on a simple parabolic band example as well as silicon. The positive Seebeck coefficient of lithium is reproduced in an example of going beyond the constant relaxation time approximation. Program Title: BoltzTraP2 Program Files doi: http://dx.doi.org/10.17632/bzb9byx8g8.1 Licensing provisions: GPLv3 Programming language: Python and C++ External routines/libraries: NumPy, SciPy, Matplotlib, spglib, ase, fftw, VTK, netCDF4, Eigen Nature of problem: Calculating the transport coefficients using the linearized Boltzmann transport equation within the relaxation time approximation. Solution method: Smoothed Fourier interpolation