Crowded solutions of densely branched polymers with starlike architecture undergo areversible gelation upon heating. This phenomenon is characterized by slow kinetics and is attributed tothe formation of clusters causing a partial dynamic arrest of the swollen interpenetrating spheres athigh temperatures. A kinetic pseudo-phase diagram of gelation temperature against the effective volumefraction is constructed. Stars with different functionalities exhibit a different sol−gel boundary; smalldifferences in the internal structure of the stars (regular with spherical dense core vs irregular withnonspherical core but spherical overall shape) are presumably responsible for these differences. Thermalgelation is proposed as an alternative route to jamming of soft materials.