Bacterial sensors, based on ligand-mediated genetic control systems, are promising for on-site chemical detection because sensing targets and generating signals do not require costly instrumentation. Here, we have constructed intragenic synthetic riboswitches without relying on high-throughput screening and demonstrated that the riboswitches can be harnessed to develop bacterial sensors displaying readily visible reporter signals in response to theophylline. In vivo imaging using the riboswitch showed target-specific changes in the expression of a green fluorescence protein reporter, which was visible even to the naked eye.