Abstract. Revianti S, Andriani D, Parisihni K, Wahjuningsih E, Widyastuti. 2020. Effectiveness of oral irrigation with an extract of green microalga Nannochloropsis oculata as an anti-inflammatory in rats infected with Aggregatibacter actinomycetemcomitans. Biodiversitas 21: 2977-2981. Aggressive periodontitis is strongly correlated with Aggregatibacter actinomycetemcomitans, a periodontopathic bacterium, which releases endotoxins and lipopolysaccharides (LPS). LPS acts as a stimulus to a variety of host cells that can promote the expression of pro-inflammatory cytokines, such as the tumor necrosis factor-α (TNF-α), in periodontal disease. The marine green microalga Nannochloropsis oculata has anti-inflammatory properties. This study aimed to examine the anti-inflammatory efficacy of oral irrigation with N. oculata extract against periodontitis induced by A. actinomycetemcomitans in a rat model. Twenty-four male Wistar rats were divided randomly into four groups (n = 6). First control group was without any treatment, the second to fourth groups were infected with A. actinomycetemcomitans and then orally irrigated with N. oculata extract at concentrations of 2.375%, 2.5%, and 2.625%. The mandibles of the euthanized rats were hemisected to measure the expression of TNF-α, interleukin-10 (IL-10, an anti-inflammatory cytokine), and osteoprotegerin (OPG) by immunohistochemistry. The first group (control) showed significantly higher expression of TNF-α and significantly lower expression of IL-10 and OPG compared to the other groups. Oral irrigation with N. oculata extracts reduced the expression of TNF-α and enhanced the expression of IL-10 and OPG in rats infected with A. actinomycetemcomitans. The effective concentration of N. oculata extract as an anti-inflammatory for oral irrigation was 2.375%.