The current paper describes the use of a sodium alginate stabilized magnetite (AFe) nanocomposite to encapsulate amoxicillin (AMX) as an antibacterial therapy against gram-positive and gram-negative bacteria, Listeria monocytogenes and Salmonella typhi, respectively. The use of alginate provides various functional groups and expanded active sites for the composite to adsorb amoxicillin via stable electrostatic interactions. A variety of imaging and spectroscopic techniques were used to analyze the as-synthesized alginate-capped magnetite (AFe) before and after composition with amoxicillin (AMX). The results of percentage loading capacity and encapsulation efficiency suggest that significant active contents of AMX were adsorbed onto AFe to afford alginate stabilized magnetite-loaded amoxicillin (AFeAMX) during loading experiments, and that these active components were released in two stages: first, via an initial burst lasting for 2 h, followed by a continuous and sustained release of AMX lasting over 48 h. The in vitro release profiles show that 86.4% and 74.6% of AMX are respectively released from the formulation at pH 6.2 and 7.4 after 96 h. Antimicrobial efficacy of AFe composited with AMX is more pronounced against L. monocytogenes, with higher inhibitory growth effect at lower AFeAMX dosages than against gram-negative bacterium of S. typhi. The results suggest that gram-negative bacteria had higher antimicrobial resistance to the amoxicillin-loaded alginate-magnetite composite than gram-positive bacteria.