This paper presents the design and development of a low-cost and low-profile sensor suitable for measuring Photosynthetic Active Radiation (PAR), which is defined as the photon flux density in 400 to 700 nm wavelength range. The sensor uses a silicon photodiode along with a bandpass optical filter and a diffuser housed inside a 3D-printed enclosure. A prototype of the sensor was built and tested with a halogen light source. Its performance was compared with research-grade commercially available PAR sensors and with a spectroradiometer. It was found that the error of the sensor depends on light intensity being the most accurate when the light source was set to maximum intensity. The cost of the developed sensor is much lower than research-grade commercially available solutions making it suitable for agricultural applications where cost is an important factor.