Gold nanoparticles are exciting materials because of their potential applications in optics, electronics, biomedical, and pharmaceutical fields. In recent years, environmentally friendly, low-cost biosynthesis methods with bio-applicable features have continued to be developed for the synthesis of gold nanoparticles. In the present study, an actinobacterial strain was isolated from the Petrosia ficiformis (Poiret 1798) sponge, which was collected from a marine environment, and the gold nanoparticle synthesis was performed for the first time from the bacteria type belonging to the Citricoccus genus. The synthesis conditions were optimized using the Box-Behnken experimental design, with a statistical method that included three independent variables (temperature, time, and mixture ratio) to affect the synthesis at three levels (+1, 0, and -1). Accordingly, the conditions proposed for the biosynthesis of gold nanoparticles at the maximum optical density values that are specific for the Citricoccus sp. K1D109 strain were estimated as 35 degrees C temperature, 24 h, and 1/5 mixture ratio (cell-free extract/HAuCl4 center dot 3H(2)O). When recommended conditions were applied, it was determined that the maximum absorbance of the synthesized gold nanoparticles is 1.258 at 545 nm, and their sizes are in the range of 25-65 nm, according to transmission electron microscopy (TEM) data.