The mass of 31 Cl has been measured with the JYFLTRAP double Penning trap mass spectrometer at the Ion Guide Isotope Separator On-Line (IGISOL) facility. The determined mass-excess value, −7034.7(34) keV, is 15 times more precise than in the Atomic Mass Evaluation 2012. The quadratic form of the isobaric multiplet mass equation for the T = 3/2 quartet at A = 31 fails (χ 2 n = 11.6) and a non-zero cubic term, d = −3.5(11) keV, is obtained when the new mass value is adopted. 31 Cl has been found to be less proton-bound with a proton separation energy of Sp = 265(4) keV. Energies for the excited states in 31 Cl and the photodisintegration rate on 31 Cl have been determined with significantly improved precision using the new Sp value. The improved photodisintegration rate helps to constrain astrophysical conditions where 30 S can act as a waiting point in the rapid proton capture process in type I x-ray bursts.