In this study, we aimed to synthesize silver nanoparticles containing fucans from Dictyota mertensii (Martius) Kützing using an environmentally friendly method and to characterize their structure as well as antiproliferative, immunomodulatory, and antibacterial effects. Fucan-coated silver nanoparticles (FN) were characterized by Fourier-transform infrared analysis, dynamic light scattering, zeta potential, atomic force microscopy, energy dispersive X-ray spectroscopy, and inductively coupled plasma emission spectrometry. They were evaluated for their effect on cell viability, minimum inhibitory bactericidal concentration, and release of nitric oxide and cytokines. The FN were successfully synthesized using an environmentally friendly method. They were size-stable for 16 months, of a spherical shape, negative charge (−19.1 mV), and an average size of 103.3 ± 43 nm. They were able to inhibit the proliferation of the melanoma tumor cell line B16F10 (60%). In addition, they had immunomodulatory properties: they caused an up to 7000-fold increase in the release of nitric oxide and cytokines (IL-10; IL-6 and TNF-α) up to 7000 times. In addition, the FN showed inhibitory effect on Gram-positive and -negative bacteria, with MIC values of 50 µg/mL. Overall, the data showed that FN are nanoparticles with the potential to be used as antitumor, immunomodulatory, and antibacterial agents.