Recent instrumentation projects have allocated resources to develop codes for simulating astronomical images. Physics-based models are essential for understanding telescope, instrument, and environmental systematics in observations. A deep understanding of these systematics is especially important in the context of weak gravitational lensing, galaxy morphology, and other sensitive measurements. We present an adaptation of a physics-based ab initio image simulator: the photon simulator (PhoSim). We modify PhoSim for use with the near-infrared camera (NIRCam)-the primary imaging instrument aboard the James Webb Space Telescope. This photon Monte Carlo code replicates the observational catalog, telescope and camera optics, detector physics, and readout modes/electronics. Importantly, PhoSim-NIRCam simulates both geometric aberration and diffraction across the field of view. Full field- and wavelength-dependent point spread functions are presented. Simulated images of an extragalactic field are presented. Extensive validation is planned during in-orbit commissioning. (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)