Marine organisms produce numerous secondary metabolites that exhibit a wide range of biological activities, which have applications in pharmaceutical research. Numerous secondary metabolites have been discovered from various marine organisms and studied for their chemical and biological properties. Among the secondary metabolites of marine organisms, alkaloids constitute a versatile group of bioactive natural products with promising bio-activities. Several alkaloids, such as pyridoacridines, pyrroles, bisindole, isoquinolines, quinolizidines and bromotyrosines, etc., to name a few, have been isolated from marine organisms. The chemical diversity and bio-activities of marine alkaloids are reported in several research and review articles. Quinolizidine alkaloids (QAs) are a group of compounds that possess either a quinolizidine ring or its derivatives. They are isolated from terrestrial plants, animals and also from numerous marine organisms, such as sponges, tunicates, fungus, etc. Biological activities exhibited by QAs include ichthyotoxicity, chemical defense, antimicrobial, antiviral, and inhibition of nicotinic acetylcholine receptors. In the past years, a few scattered reviews appeared on the isolation of QAs from natural sources, mostly from terrestrial sources, but the reports skipped several QAs of marine origin. This chapter presents a comprehensive review of various quinolizidine and bis-1-oxaquinolizidine alkaloids isolated from marine organisms, detailing their chemical structures and reported biological properties. Further, the chapter highlighted synthesis of some marine-derived QAs, namely, petrosins, xestospongins, clavepictines, pictamine, citrinadins A and B.