Polyhydroxyalkanoate (PHA) production using halophilic bacteria has been revisited because less severe operational conditions with respect to sterility can be applied, also alleviating production costs. Halomonas boliviensis was selected because it is a moderate halophile able to grow and attain high poly-3-hydroxybutyrate (P3HB) contents under 5–45 g/L NaCl concentrations, conditions that discourage microbial contamination. Industrial residues of the red alga Gelidium corneum after agar extraction were used as sugar platform to reduce costs associated with the carbon source. These residues still comprise a high carbohydrate content (30–40% w/w) of mainly cellulose, and their hydrolysates can be used as substrates for the bioproduction of value-added products. Preliminary assays using glucose were carried out to determine the best conditions for growth and P3HB production by H. boliviensis in bioreactor fed-batch cultivations. Two strategies were addressed, namely nitrogen or phosphorus limitation, to promote polymer accumulation. Similar P3HB cell contents of 50% (gpolymer/gCDW) and yields YP3HB/glucose of 0.11–0.15 g polymer/g glucose were attained under both conditions. However, higher specific productivities were reached under P-limitation, and thus, this strategy was adopted in the subsequent study. Two organic acids, resulting from glucose metabolism, were identified to be gluconic and 2-oxoglutaric acid. Reducing the oxygen concentration in the cultivation medium to 5% sat was found to minimize organic acid production and enhance the yield of polymer on sugar to 0.20 gP3HB/gglucose. Finally, fed-batch cultivations using G. corneum hydrolysates as the only C-source achieved an overall volumetric productivity of 0.47 g/(L.h), 40% polymer accumulation, and negligible gluconic acid production.