Organobromine (Br_org) compounds, commonly recognized as persistent, toxic anthropogenic pollutants, are also produced naturally in terrestrial and marine systems. Several enzymatic and abiotic bromination mechanisms have been identified, as well as an array of natural Br_org molecules associated with various marine organisms. The fate of the carbon‐bromine functionality in the marine environment, however, remains largely unexplored. Oceanographic studies have noted an association between bromine (Br) and organic carbon (C_org) in marine sediments. Even so, there has been no direct chemical evidence that Br in the sediments exists in a stable form apart from inorganic bromide (Br_inorg), which is widely presumed conservative in marine systems. To investigate the scope of natural Br_org production and its fate in the environment, we probed Br distribution and speciation in estuarine and marine sediments using in situ X‐ray spectroscopy and spectromicroscopy. We show that Br_org is ubiquitous throughout diverse sedimentary environments, occurring in correlation with C_org and metals such as Fe, Ca, and Zn. Analysis of sinking particulate carbon from the seawater column links the Br_org observed in sediments to biologically produced Br_org compounds that persist through humification of natural organic matter (NOM). Br speciation varies with sediment depth, revealing biogeochemical cycling of Br between organic and inorganic forms as part of the burial and degradation of NOM. These findings illuminate the chemistry behind the association of Br with C_org in marine sediments and cast doubt on the paradigmatic classification of Br as a conservative element in seawater systems.