AbstractExcited states in the extremely neutron-deficient nucleus $^{172}{\mathrm {Pt}}$172Pt were populated via $^{96}\mathrm {Ru}(^{78}\mathrm {Kr},2p)$96Ru(78Kr,2p) and $^{92}\mathrm {Mo}(^{83}\mathrm {Kr},3n)$92Mo(83Kr,3n) reactions. The level scheme has been extended up to an excitation energy of $≈ 5~\hbox {MeV}$≈5MeV and tentative spin-parity assignments up to $I^π = 18^+$Iπ=18+. Linear polarization and angular distribution measurements were used to determine the electromagnetic E1 character of the dipole transitions connecting the positive-parity ground-state band with an excited side-band, firmly establishing it as a negative-parity band. The lowest member of this negative-parity structure was firmly assigned spin-parity $3^-$3-. In addition, we observed an E3 transition from this $3^-$3- state to the ground state, providing direct evidence for octupole collectivity in $^{172}{\mathrm {Pt}}$172Pt. Large-scale shell model (LSSM) and total Routhian surface (TRS) calculations have been performed, supporting the interpretation of the $3^-$3- state as a collective octupole-vibrational state.