The ‘Burmese amber’ (~99 Ma, Myanmar), widely known for exquisite preservation of a mid-Cretaceous terrestrial faunas including dinosaur remains and birds, also yields marine fossils. Ostracods are usually small (0.5–2 mm), with well-calcified carapaces that provide an excellent fossil record extending to at least the Ordovician (~485 million years ago), but they are rarely encountered in amber. The few examples known derive from Eocene and especially Miocene amber deposits. Our first finding of a valve of a ‘gigantic’ (12.9 mm) marine ostracod in Burmese amber effectively doubles the age of the ostracod amber record but also offers the first representative of the fully marine Myodocopa, a weakly calcified group with a poor fossil record. Subsequent resin flows in the amber specimen contain terrestrial arthropods (spider fragments) and insect frass. These features resolve an enigmatic taphonomic pathway which seems to be much less elaborate than scenarios for inclusion of aquatic (freshwater mostly) organisms in amber proposed before, and support a marginal marine setting for resin production. The combination of marine and terrestrial resin flows may have been brought about by variation in water levels, a mechanism proposed in the study of modern marine organisms preserved in resin.