Acid whey from Greek-style yogurt (GAW) is a byproduct of interest in the dairy industry, due to the boom in its production and its potential negative environmental impact. Concentration of GAW solids can facilitate their use, storage, and transportation. This study focuses on achieving a high concentration factor of GAW using a combination of reverse osmosis (RO) and forward osmosis (FO), to avoid challenges and reduce costs associated with its thermal concentration. GAW from a NY State plant was first concentrated at 5 to 13°C using a RO rig (Osmonics, WI) with a spiral-wound aromatic polyamide membrane (Dow, NY), then by FO at 4 to 10°C using a FO unit (Ederna, France) with a modified spiral-wound cellulose triacetate membrane, and potassium lactate 60° Brix as draw solution. Experiments were conducted at least in triplicate, using different batches of GAW. The water fluxes were determined gravimetrically, and total specific energy for each process was calculated. The electrical energy required for pumping was calculated using the measured current and voltage in the system. For RO, the cooling energy was also determined. For FO, the energy required for regenerating the osmotic agent was estimated based on the specifications of the evaporator (Evaled, Italy). Fresh GAW, ranging in concentration from 5.9 to 6.8° Brix, was concentrated by RO to 19.1 ± 2.0° Brix. Part of this concentrate was then concentrated by FO to 38.1 ± 4.7° Brix. The water flux for RO dropped from 28.8 ± 0.5 L/m²h to 14.7 ± 1.9 L/m²h, and for FO from 3.7 ± 0.5 L/m²h to 2.5 ± 0.2 L/m²h. The total specific energy consumption was 0.29 ± 0.04 kWh/kg water for RO and 0.65 ± 0.06 kWh/kg water for FO. For FO, about half of the required energy pertains to the draw solution regeneration. This work will provide guidance on how to combine RO and FO to produce high quality GAW concentrates in a cost-effective manner. The energy calculations provided here serve as an initial estimate, and scaling to commercial scale can bring them down due to large volumes, continuous operation, and more efficient processing.