FFTJet: A Package for Multiresolution Particle Jet Reconstruction in the Fourier Domain. This article describes the FFTJet software package designed to perform jet reconstruction in the analysis of high energy physics experimental data. A two-stage approach is adopted in which pattern recognition is performed first, utilizing multiresolution filtering techniques in the frequency domain. Jet energy reconstruction follows, conditional upon the choice of signal topology. The method is efficient, global, collinear and infrared safe, and allows the user to identify and avoid the event topology bifurcation points when energy reconstruction is performed.