Botanical extracts, including essential oils, are promising alternatives to synthetic insecticides for pest control. In this study, we evaluated the fumigant toxicity of an essential oil extracted from Thymus capitatus (L.) Hoffmanns. & Link against the citrus mealybug, Planococcus citri Risso (Hemiptera: Pseudococcidae), and its coccinellid predator Cryptolaemus montrouzieri Mulsant (Coleoptera: Coccinellidae) under laboratory conditions. Gas chromatography–mass spectrometry analysis indicated that the major chemical compounds identified from T. capitatus were carvacrol (65.15%), followed by p-cymene (11.79%) and γterpinene (7.48%). High mortality levels were registered for P. citri larvae (up to 100%) and adults (up to 96%) when exposed to the tested essential oil. The median lethal concentration values calculated for P. citri adults were higher than for larvae. Thymus capitatus essential oil applied at 10 and 20 µL/L_air showed high toxicity towards C. montrouzieri adults. These results highlighted the efficacy of T. capitatus essential oil as a promising tool to control P. citri in Tunisia. However, the adverse effects of this oil towards C. montrouzieri should be taken into consideration to enhance its practical implication in integrated pest management.