Heme compounds have been detected in particulate organic matter in the oceans, but very little is known regarding the transformations of these molecules in the marine environment. The Roseobacter clade is a biogeochemically important bacterial lineage in marine systems. Prior studies have shown that nearly 50% of Roseobacter isolate genomes contain putative heme uptake and utilization loci. Here we demonstrate that one isolate, Ruegeria sp. TM1040, can maintain growth using heme b as the Fe source. Further, we show that insertional inactivation of a putative heme uptake TonB dependent outer membrane receptor in Ruegeria sp. TM1040 results in complete inability to utilize heme b as an Fe source. This is the first functional characterization of an outer membrane heme receptor in a non-pathogenic marine organism. As heme is fundamental to diverse enzymes, it could be advantageous for microbes in the iron-poor oceans to have dedicated systems for assimilating heme directly. We propose that intact heme-containing molecules are a component of the bioavailable Fe pool for marine heterotrophic bacteria, and much like vitamins can be considered “growth factors” in the sea.