We successfully expressed and crystallized the dddC gene (Uniprot code G5CZI2) product from the Gram-negative marine bacterium Oceanimonas doudoroffii. The DddC is a methylmalonate-semialdehyde dehydrogenase (OdMMSDH) enzyme that is involved in dimethyl sulfonio propionate (DMSP) catabolism. DMSP is produced by marine phytoplankton and macroalgae, and is enzymatically metabolized into dimethylsulfide (DMS) or 3-methiolpropionate. DMS is a major source of sulfur gases in marine environments and induces cloud nuclei condensation. As a result of DMSP catabolism, DMS plays an important role in the global sulfur cycle and climate change. In order to understand the structural details of OdMMSDH, the recombinant protein was over-expressed in Escherichia coli and successfully crystallized in 21% (w/v) PEG 3350 and 0.2 M potassium sodium tartrate, pH 7.5. Furthermore, a complete native diffraction data set was collected up to 2.9 Å resolution and processed in the P21212 space group with unit-cell parameters a = 156.7, b = 160.3, and c = 238.9 Å. Phase information was obtained by molecular replacement, and structure refinement and model building are in progress. To better understand the enzymatic mechanisms of MMSDH from O. doudoroffii (OdMMSDH), we have performed biochemical and structural studies. As a first step towards its structural characterization, we here report the over-expression, purification, crystallization, and preliminary X-ray diffraction analysis of OdMMSDH.