Production rates of bromoform (CHBr₃), methylene bromide (CH₂Br₂), and methyl iodide (CH₃I) were measured in the laboratory for 11 species of marine macroalgae. Production rates of the volatile bromomethanes extrapolated to a global scale suggest that marine macroalgae produce 2 × 10¹¹ g Br yr⁻¹(1 × 10⁹ mol Br yr⁻¹), 98% of which is bromoform. Laminarians (kelps) produce 61% of this organic Br. These calculations suggest that marine macroalgae are important in the biogeochemical cycling of Br. Seawater concentrations of CHBr₃, CH₂Br₂, and CH₃I were determined from various southern California coastal locales. High concentrations were measured in seawater from the canopy and the bottom of a dense bed of Macrocystis as compared to other sites. Surface seawater concentrations of these halomethanes showed a strong cross‐shore gradient with the highest concentration in the kelp canopy and the lowest at 5 km offshore. Seawater adjacent to decaying macroalgae on the bottom of a submarine canyon was not enriched in halomethanes relative to surface water. Water exiting a productive estuary was enriched only with CH₂Br₂, although two algal species that are abundant there (Ulva and Enteromorpha) showed high laboratory production rates of both CHBr₃ and CH₂Br₂.