Isoimperatorin (IMP), an active natural furocoumarin, has numerous pharmacologic effects, including anti-inflammatory, analgesic, antispasmodic, and anticancer activities. This study aimed to evaluate the preventive activity of IMP in an ovalbumin (OVA)-induced murine model of asthma and to investigate its possible molecular mechanisms. Female BALB/c mice were sensitized on days 0 and 14 via intraperitoneal injection of 20 μg OVA. On days 21–23 after the initial sensitization, the mice received an airway challenge with OVA (1% w/v in PBS) for 1 h; meanwhile, IMP (10 or 30 mg/kg once daily) was administered by gavage on days 18–23. Our results revealed that IMP significantly lowered the productions of interleukin (IL)-4, IL-5, IL-13, eotaxin, and immunoglobulin (Ig)E in bronchoalveolar lavage fluid (BALF), plasma, or lung tissues. Histological studies showed that IMP inhibited OVA-induced inflammatory cell infiltration and mucus production in the respiratory tract. In addition, pretreatment with IMP suppressed the activation of p38 mitogen-activated protein kinase (p38 MAPK), extracellular-signal-regulated kinases 1/2 (ERK1/2), and nuclear factor-κB (NF-κB). Together, these results suggest that IMP effectively inhibits airway inflammation and mucus hypersecretion by downregulating the levels of Th2 cytokines and inhibiting NF-κB and MAPK pathways.