Replacing the large amounts of chemical fertilizer with nutrients from waste or residual streams is an important factor to make microalgal production more sustainable, cost-effective, and part of a circular bioeconomy. This is the first study to investigate insect frass as a potential nutrient source for microalgal cultivation, and its effect on the protein content of the microalgae Chlorella vulgaris. When grown on nutrients extracted from insect frass, C. vulgaris grew equally well and showed similar high protein content (40 % of the dry weight) as when grown in a control medium based on commercial fertilizers. The nitrogen in the frass media consisted predominantly of organic nitrogen compounds, of which 71–78 % could be consumed by the microalgae. While the presence of dissolved organic carbon in the insect frass promoted the growth of algae-associated bacteria, microalgal performance was not affected.