High water demand is a major challenge for the algae industry, so cultivating algae in wastewater can have the double benefit of biomass production and water remediation. The use of landfill leachate (LL), which is wastewater generated in landfills, was investigated to grow the microalga Picochlorum oculatum in a novel horizontal bioreactor (HBR), a low-cost modular cultivation system that reduces water evaporation and contamination risk thanks to its enclosed design. Pilot-scale (150 L) and commercial-scale (2000 L) HBRs that were operated outdoors in Florida using LL in batch and semi-continuous modes generated high cell density cultures (1.7·109 cells mL-1) and reached up to 1.9 g L-1 of dry biomass suitable for biofuel production. Demonstrating the ability of ample non-potable water sources, such as LL, to support algae cultivation is essential for improving the sustainability and cost-effectiveness of commercial algal biofuels and bioproducts, as freshwater resources become increasingly scarce.