Green synthesis of silver nanoparticles (AgNPs) is an alternative to the conventional synthesis procedures which includes physical and chemical methods mostly requiring toxic chemicals, energy, high temperature, and pressure. This study involves biosynthesis of AgNPs using mangroves, a salt-tolerant tidal vegetation with very unique morphology and unusual physiological processes. Our study reports the first ever use of three mangrove plants from Indian Sundarban for bioreduction, namely Avicennia alba, Sonneratia caseolaris, and Sonneratia apetela. The biosynthesized AgNPs were characterized by UV–vis spectroscope, particle size analyzer, scanning electron microscope, transmission electron microscope, energy dispersive X-ray spectrometer, and atomic force microscope. Antimicrobial activities of these AgNPs were assessed against Escherichia coli, Agrobacterium tumefaciens, Streptococcus mutans, Staphylococcus aureus, Tricophyton rubrum, and Aspergillus flavus. Biosynthesized AgNPs showed absorption maxima between 419 and 448 nm which corresponds to their respective surface plasmon resonance. Previous biosynthesis of AgNPs using mangrove plants have reported 60–110 nm average particle size, whereas in our study, S. caseolaris was the most potent bioreductant which synthesized AgNPs with average diameter (D90%) of 18.3 nm. The particles exhibited considerable antimicrobial activities against all six microorganisms. AgNPs synthesized by S. caseolaris using 5 ppm AgNO3 showed the most significant activity with maximum zone of inhibition (13.5 ± 0.8 mm) against E. coli. © 2015, Springer Science+Business Media New York.