The unique perspective that microalgae biomass presents for bioenergy production is currently being strongly considered. This type of biomass production involves large amounts of nutrients, due to nitrogen and phosphorous fertilizers, which impose production limitations. A viable alternative to fertilizers is wastewater, rich in essential nutrients (carbon, nitrogen, phosphorus, potassium). Therefore, Arthrospira platensis was cultivated in 150 mL photobioreactors with 70% (v/v) with the wastewater from a dairy industry, under a regime of light:dark cycles (12 h:12 h), with an irradiance of 140 μmol m−2 s−1 photon. The discontinuous cultures were inoculated with an average concentration of chlorophyll-a of 13.19 ± 0.19 mg L−1. High biomass productivity was achieved in the cultures with wastewater from the dairy industry (1.1 ± 0.02 g L−1 d−1). This biomass was subjected to thermal and physical treatments, to be used in co-digestion with cattle manure. Co-digestion was carried out in a mesophilic regime (35 °C) with a C: N ratio of 19:1, reaching a high methane yield of 482.54 ± 8.27 mL of CH4 g−1 volatile solids (VS), compared with control (cattle manure). The results demonstrate the effectiveness of the use of cyanobacterial biomass grown in wastewater to obtain bioenergy.