The highly photosensitive characteristics of organic thin-film transistors (OTFTs) made using soluble star-shaped oligothiophenes with four-armed π-conjugation paths, 4(HPBT)-benzene and 4(HP3T)-benzene molecules having a relatively high quantum yield, are reported. 4(HPBT)-benzene-based organic phototransistors (OPTs) exhibited high photosensitivity (∼2500–4300 A W−1) even with low optical powers (∼6.8–30 µW cm−2) at zero gate bias. The measured photosensitivity of the devices was much higher than that of inorganic single-crystal Si-based phototransistors, as well as that of other OPTs reported earlier. With the highly photosensitive characteristics of the 4(HPBT)-benzene-based OPTs, a high ratio of the on and off current switching of ∼4 × 104 with low optical power and low gate bias was observed. The slow relaxation of the photoinduced charges and charge-trapping phenomena at the interface could lead to a reproducible memory operation for 4(HPBT)-benzene-based OPTs.