We report on the fabrication and characterization of organic phototransistors (OPTs) based on fluorescent nanocrystals assembled from a simple organic dye molecule (N,N′-bis(2,4-dimethylpent-3-yl)perylene-3,4:9,10-tetracarboxylic diimide, DMP-PDI). The OPT active layer is based on DMP-PDI nanocrystals assembled in aqueous solution or within polymer films. Despite the absence of any π-overlap, the nanocrystals show mobilities as high as (5 ± 1) × 10⁻³ cm² V⁻¹ s⁻¹ in polymer films, which is due to imide/π-core noncovalent interactions leading to substantial electronic coupling as revealed by computational studies. The OPTs strongly respond to white light irradiation, resulting in a decrease in threshold voltage by as much as 40 V. OPTs based on nanocrystals assembled within polymer films have threshold voltages close to 0 V upon illumination and a high photo/dark current ratio (P = 4 × 10³). We show that the organic crystals lacking π-overlap mediate charge mobility and are advantageous as active layers for OPTs due to diminished nonradiative decay.