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Microalgal cultivation for biodiesel and feed requires recycled nutrient resources for a sustainable long-term operation. Whey permeate (WP) from dairy processing contains high organic load (lactose, oils, and proteins) and nitrogen (resources tested for microalgal cultivation) and organic phosphorus (P) that has not yet been tested as a P source for microalgal cultivation. We explored the potential of green algae strains (brackish) and polyculture (freshwater) in exploiting P from WP added to a medium based on either seawater (7 psu) or landfill leachate. Both strains showed a capacity of using organic P in WP with equal growth rates (0.94–1.12 d−1) compared with chemical phosphate treatments (0.88–1.07 d−1). The polyculture had comparable growth rate (0.25–0.57 d−1) and biomass yield (152.1–357.5 mg L−1) and similar or higher nutrient removal rate in the leachate–WP medium (1.3–6.4 mg L−1 day−1 nitrogen, 0.2–1.1 mg L−1 day−1 P) compared with the leachate–chemical phosphate medium (1.2–4.7 mg L−1 day−1 nitrogen, 0.3–1.4 mg L−1 day−1 P). This study showed that WP is a suitable P source for microalgal cultivation over a range of salinities. To date, this is the first study demonstrating that raw WP can replace mineral P fertilizer for algal cultivation.