Background: Kelussia odoratissima Mozaff. (Apiaceae) is an edible, indigenous, and ethnomedicinal plant that grows only in Iran. Although antioxidant and anti-inflammatory properties of K. odoratissima have been reported, cytotoxic activity of this plant has not been investigated previously. Objective: This study aims to evaluate the cytotoxicity of K. odoratissima leaf extract against a panel of human cancer cell lines. A secondary aim was to perform a phytochemical analysis of the plant’s leaf oil. Methods: To extract the plant oil, dried leaves were subjected to hydrodistillation using a Clevenger-type apparatus for up to 3 hours. For the phytochemical analysis, essential oil was subjected to gas chromatography/mass spectrometry. Plant extraction was performed by macerating leaf powder of K. odoratissima (50 g) in 70% methanol (500 mL) at room temperature (25–28°C) for 24 hours. To perform cytotoxicity assays, methanolic extract of K. odoratissima was tested against a panel of cell lines including MDA-MB468 (human breast cancer cell line), K562 (human leukemia cell line), SKOV3 (human ovarian cancer cell line), Y79 (human eye cancer cell line), A549 (lung cancer cell line), and HEK 293 (normal human embryonic kidney cell line). Results: Gas chromatography/mass spectrometry analysis revealed that sesquiterpens are dominant volatile components of the plant, followed by phthalides comprising 3-butyldine phthalide and 3-n-butyl phthalide, the latter compound being the major component of the leaf oil (25.1%). The leaf extract showed selective and dose-dependent cytotoxicity against MDA-MB468, K562, SKOV3, Y79, and A549 cancer cell lines with IC50 values (concentration that inhibits cell growth by 50%) of 85 μg/mL, 70 μg/mL, 120 μg/mL, 82 μg/mL, and145 μg/mL, respectively. Conclusions: The present results suggest a direct cytotoxic activity of K. odoratissima leaf extract against human cancer cell lines. This activity of K. odoratissima may find application in combination with traditional herbal medicines to develop a new anticancer pharmacopuncture therapy.