Conventional marine-degradable plastics can remain in seawater for several years without losing their strength and stability, creating major environmental problems and threatening marine life. In this study, we prepared a cellulose nanofiber-reinforced starch film with pH-responsive weakening in seawater and marine-microbial degradability. The TEMPO-oxidized cellulose nanofiber (TCNF) was blended with hydroxypropyl starch (HPS) and di-aldehyde starch to develop a TCNF/modified starch film. pH-responsive swelling and weakening were performed in freshwater and seawater. Moreover, marine microbial degradability was evaluated in an artificial marine environment. TCNF/modified starch film rapidly loses up to 80 % of its strength in seawater and has higher marine microbial growth and degradability than TCNF film. The TCNF/HPS film demonstrates pH-responsive weakening with adequate microbial degradability. Therefore, it is a potential next-generation packaging material that can help address the problem of floating debris.