Marine biopolymer-based nanomaterials are one of the most active research areas in recent decades for theranostic applications. Marine biopolymers are interesting biomaterials for clinical applications because of their good biocompatibility, biodegradability, inexpensiveness, abundance, stability, ease of surface modification, and nontoxic nature. New nanoparticles in development are coated with marine polymers to combine therapeutic and diagnostic (theranostic) applications because of the strongly enhanced absorption and scattering in near-infrared (NIR) regions. In this review, the use of marine biopolymer-based nanomaterials for theranostic applications is evaluated, addressing potential applications in drug delivery, photothermal therapy (PTT), photodynamic therapy (PDT), hyperthermia therapy, photoacoustic imaging (PAI), magnetic resonance imaging (MRI), and computed tomography (CT). In addition, the most recent progress in the biocompatibility of marine biopolymer-based nanomaterials in vitro and in vivo are discussed, along with a promising future scope for the treatment of major life-threatening diseases such as cancer.