Photothermal therapy makes use of photothermal sensitizers and laser light to thermally ablate diseased tissues. Porphysome nanoparticles offer a nontoxic alternative to inorganic nanocrystals for the efficient conversion of light into heat. Mn(3+) ions were incorporated directly into the building blocks of our porphysome nanoparticles, thus imparting MRI sensitivity while simultaneously improving photostability and maintaining high photothermal efficiency. Mn porphysomes are as photothermally effective as free-base porphysomes and can rival gadolinium diethylenetriaminepentaacetate (Gd-DTPA) for MRI contrast generation. Their MRI contrast generation, photothermal efficiency, and photostability are unprecedented for an all-organic nanoparticle composed of a single functional component.