This article introduces the concept of photothermal isosbesticity in plasmonics. In analogy with absorbance spectroscopy, this concept designates nanostructures that feature an invariance of their temperature increase upon varying the illumination polarization angle. We show that nontrivial (i.e., non-centrosymmetric) isosbestic nanostructures exist and prove valuable when the optical near-field intensity remains, on the contrary, highly dependent on the illumination polarization. The concept is introduced with the case of a sphere-dimer, where the conditions for isosbesticity can be derived analytically. The cases of a spheroid and a disc-dimer are also studied in order to draw a general theory and explain how isosbesticity conditions can be obtained from the visible to the infrared range. Nontrivial isosbestic plasmonic nanostructures represent powerful systems to elucidate the origin (thermal or optical) of mechanisms involved in plasmonics-assisted nanochemistry, liquid–gas phase transition, or heat-assisted magnetic recording.