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A green, rapid and controllable synthesis of gold nanoparticles (Au NPs) was achieved using the mulberry fruit (MF) extract as reducing and capping agent at room temperature. Obtained Au NPs were characterized by ultraviolet–visible spectrum, transmission electron microscopy, energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses. It could be found that Au NPs had a small size of 4.2–20.1 nm, face-centered cubic crystalline structure and homogeneous dispersion, indicating strong reducing and capping capacities of the MF extract. Meanwhile, the morphology (quasi-sphere/sphere) of Au NPs could be controlled only by adjusting the MF extract concentration. Furthermore, based on the 4-nitrophenol (4-NP) reduction reaction, as-prepared Au NPs exhibited excellent photocatalytic activity and stability in 30 days. Under sunlight irradiation, 4-NP could be completely reduced in 4 min and its rate constant could reach up to 0.91 min−1. And the rate constant could be linearly adjusted by the MF extract concentration or the amount of Au NPs hydrosol. In addition, it could also be found that the light source with short wavelength could obviously elevate the photocatalytic activity of Au NPs, attributed to the hot-electron effect. Clearly, as-obtained Au NPs can have great potential applications in wastewater treatment. [ABSTRACT FROM AUTHOR]