An algorithm for computing the incomplete gamma function γ * (a, z) for real values of the parameter a and negative real values of the argument z is presented. The algorithm combines the use of series expansions, Poincaré-type expansions, uniform asymptotic expansions and recurrence relations, depending on the parameter region. A relative accuracy ∼ 10 −13 in the parameter region (a, z) ∈ [−500, 500]×[−500, 0) can be obtained when computing the function γ * (a, z) with the Fortran 90 module IncgamNEG implementing the algorithm.