Here we report the identification of over 100 small-molecules activating the tumour suppressor p53 in cells. Of these, we chose to elucidate the mechanism of action of a chiral tetrahydroindazole, HZ00. By analysing a series of cellular responses to HZ00, we deduced that its active enantiomer, (R)-HZ00, inhibits dihydroorotate-dehydrogenase (DHODH). The basis for the chiral specificity of HZ05, a more potent analogue of HZ00, is revealed by the crystal structure of the (R)-HZ05/DHODH complex. In addition to the HZ series, twelve DHODH inhibitor chemotypes were also identified amongst the p53 activators listed, which suggests that DHODH is a remarkably frequent target for structurally diverse compounds. We observed during target deconvolution that HZ compounds accumulate cancer cells in S-phase, increase p53 synthesis, and synergise to reduce tumour growth in vivo when co-administered with an inhibitor of p53 degradation. We therefore propose a new strategy to promote cancer cell killing by p53 instead of its reversible cell cycle arresting effect.