Topological superconductors (SCs) are novel phases of matter with nontrivial bulk topology. They host at their boundaries and vortex cores zero-energy Majorana bound states, potentially useful in fault-tolerant quantum computation [1]. Chiral SCs [2] are particular examples of topological SCs with finite angular momentum Cooper pairs circulating around a unique chiral axis, thus spontaneously breaking time-reversal symmetry (TRS). They are rather scarce and usually feature triplet pairing: best studied examples in bulk materials are UPt<3> and Sr<2>RuO<4> proposed to be f-wave and p-wave SCs respectively, although many open questions still remain [2]. Chiral triplet SCs are, however, topologically fragile with the gapless Majorana modes weakly protected against symmetry preserving perturbations in contrast to chiral singlet SCs [3, 4]. Using muon spin relaxation (μSR) measurements, here we report that the weakly correlated pnictide compound LaPt<3>P has the two key features of a chiral SC: spontaneous magnetic fields inside the superconducting state indicating broken TRS and low-temperature linear behaviour in the superfluid density indicating line nodes in the order parameter. Using symmetry analysis, first principles band structure calculation and mean-field theory, we unambiguously establish that the superconducting ground state of LaPt<3>P is chiral d-wave singlet.