A three-piece infrared single wavelength projection lens system includes, in order from an image side to an image source side: a first lens element with a positive refractive power; a second lens element with a negative refractive power; a third lens element with a positive refractive power; and a stop disposed before an image source-side surface of the first lens element or between an image-side surface of the first lens element and an image source-side surface of the second lens element. Such arrangements can provide a three-piece infrared single wavelength projection lens system with better image sensing function.