A pyrene-modified β-cyclodextrin (pyrenecyclodextrin)-decorated single-walled carbon nanotube (SWNT) field-effect transistor (FET) device was fabricated, which can serve as a tunable photosensor to sense a fluorescent adamantyl-modified Ru complex (ADA−Ru). When the light is on (I = 40 W m−2 and λ = 280 nm), the transfer curve of the pyrenecyclodextrin-SWNT/FET device shifts toward a negative gate voltage by about 1.6 V and its sheet resistance increases quickly, indicating a charge-transfer process from the pyrenecyclodextrins to the SWNTs. In contrast, the transfer curve of the pyrenecyclodextrin-SWNT/FET device in the presence of the ADA−Ru complex shifts toward a positive gate voltage by about 1.9 V and its sheet resistance decreases slowly when the light is on (I = 40 W m−2 and λ = 490 nm), showing a charge-transfer process from the pyrenecyclodextrin-SWNT hybrids to the ADA−Ru complex. Because these photoresponse processes are recoverable following the removal of the light, the present photosensor exhibits a promising application in the area of tunable light detection.