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Lead chalcogenide quantum dots (QDs) are promising acceptors for photovoltaic devices that harness the singlet fission (SF) mechanism. The rate of singlet fission of polyacenes in the presence of QDs is a critical parameter in determining the performance of such devices. The present study demonstrates that the rates of SF in a pentacene derivative, 6,13-diphenylanthracene (DPP), are modulated by forming coaggregates with PbS QDs in aqueous dispersions. PbS QDs generally accelerate SF within DPP aggregates, and the extent of acceleration depends on the size of the QD. The average rate of SF increases from 0.074 ps −1 for DPP-only aggregates to 0.37 ps −1 within DPP-D co-aggregates for QDs with radius 2.2 nm, whereas co-aggregation with the smallest ( r = 1.6 nm) and largest ( r = 2.7 nm) QDs we tried only slightly change the SF rate. The rate variation is associated with (i) the density of surface ligands, which is influenced by the faceting of the PbS surface, and (ii) the local dielectric constant for DPP. To accelerate SF, the ligands should be dense enough to provide sufficient affinity for DPP aggregates and effectively perturb the perpendicular alignment of DPP monomers within aggregates to increase the intermolecular coupling that promotes SF, but should not be too dense so as to form a low dielectric environment that disfavors SF. The study suggests that it is critical to consider the influence of the microenvironment of the QD surface on photophysical processes when fabricating QD/organic hybrid devices.