In conducting ferromagnets, an anomalous Nernst effect-the generation of an electric voltage perpendicular to both the magnetization and an applied temperature gradient-can be driven by the nontrivial geometric structure, or Berry curvature, of the wavefunction of the electrons(1,2). Here, we report the observation of a giant anomalous Nernst effect at room temperature in the full-Heusler ferromagnet Co2MnGa, an order of magnitude larger than the previous maximum value reported for a magnetic conductor(3,4). Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions(5-7) is responsible for the observed - Tlog(T) behaviour, with T denoting the temperature, and the enhanced value of the transverse thermoelectric conductivity. The temperature dependence of the thermoelectric response in experiments and numerical calculations can be understood in terms of a quantum critical-scaling function predicted by the low-energy effective theory over more than a decade of temperatures. Moreover, the observation of an unsaturated positive longitudinal magnetoconductance, or chiral anomaly(8-10), also provides evidence for the existence of Weyl fermions(11,12) in Co2MnGa.