In multiplexed analysis, lab on a chip (LoC) devices are advantageous due to the low sample and reagent volumes required. Although optical detection is preferred for providing high sensitivity in a contactless configuration, multiplexed optical LoCs are limited by the technological complexity for integrating multiple light sources and detectors in a single device. To address this issue, we present a microfluidic-controlled optical router that enables measurement in four individual optical channels using a single light source and detector, and without movable parts. The optofluidic device is entirely fabricated in polydimethylsiloxane (PDMS) by soft-lithography, compatible with standard microfabrication technologies, enabling monolithic integration in LoCs. In the device, in-coupled light from an optical fiber is collimated by a polymeric micro-lens and guided through a set of four sequentially connected micro-chambers. When a micro-chamber is filled with water, light is transmitted to the next one. If it is empty of liquid, however, total internal reflection (TIR) occurs at the PDMS-air interface, re-directing the light to the output optical fiber. The router presents high performance, with low cross-talk (<2%) and high switching frequencies (up to 0.343 ± 0.006 Hz), and provides a stable signal for up to 91% of the switching time. With this miniaturized, low-cost, simple and robust design, we expect the current technology to be integrated in the new generation of multiplexed photonic LoCs for biomarker analysis, even at the point of care.