Alkaloids are nitrogen-containing heterocyclic compounds typically isolated from plants. They represent one of the most important types of natural products because of their large number and structural diversity and complexity. Based on their chemical core structures, alkaloids are classified as isoquinolines, quinolines, indoles, piperidine alkaloids, etc. In-depth analyses of alkaloids have revealed their antibacterial activities. To date, due to the widespread use of antibiotics, the problem of drug-resistant bacterial infections has been gradually increasing, which severely affects the clinical efficacy of antibacterial therapies and patient safety. Therefore, significant research efforts are focused on alkaloids because they represent a potentially new type of natural antibiotic with a wide antibacterial spectrum, rare adverse reactions, and a low tendency to produce drug resistance. Their main antibacterial mechanisms include inhibition of bacterial cell wall synthesis, change in cell membrane permeability, inhibition of bacterial metabolism, and inhibition of nucleic acid and protein synthesis. This article reviews recent reports about the chemical structures and the antibacterial activities and mechanisms of alkaloids. The purpose is to solve the problem of bacterial resistance and to provide a certain theoretical basis and research ideas for the development of new antibacterial drugs.