Background Hypoxic pulmonary hypertension (HPH) is initiated by acute hypoxic pulmonary vasoconstriction followed by vascular remodeling and right ventricular failure. Numerous studies have shown that long-term supplementation with docosahexaenoic acid (DHA) may have a role in preventing cardiovascular disease. However, the effects of DHA on HPH remain unclear. Extracellular signal-regulated kinase 1/2 (ERK1/2) is known to be involved in the proliferation and migration of various cell types. In this study, we determined the role of DHA in the prevention of HPH via the ERK1/2 signal pathway. Methods Rats were intragastrically administered with saline or DHA (100 mg) daily before exposure to room air or chronic hypoxia (O2 content was maintained at 10%) for 8 h a day for 3 weeks. At the end of study, we compared the right ventricular systolic pressure (RVSP), the weight ratio of right ventricular (RV) free wall to the left ventricular (LV) including the septum (S) free wall, the percent wall thickness (wt.%) of small pulmonary arteries and the area of α-smooth muscle actin (α-SMA)-positive pulmonary artery smooth muscle cells (PASMCs) in the pulmonary arteries among the rats. In vitro, the proliferation and migration of PASMCs were through an MTT assay and a Boyden chamber, respectively. The phenotype marker expression and ERK1/2 activation in the PASMCs were through real-time PCR and western blot analysis, respectively. Results Under hypoxia, DHA treatment reduced the RVSP (20.41 mm Hg ± 2.18 vs. 35.46 mm Hg ± 3.21; P < 0.05), the weight ratio of RV free wall to left ventricular and septal free wall [RV / (LV + S)] (0.27 ± 0.03 vs. 0.38 ± 0.05; P < 0.05), wt.% (17.45 ± 1.58% vs. 59.65 ± 4.59%; P < 0.05) and the percentage of SMA-positive vessels (55.42 ± 5.18% vs. 84.71 ± 6.22%; P < 0.05) compared with those of the saline-treated rats. Hypoxia promoted the proliferation, migration and phenotype switching of PASMCs in vitro. However, DHA treatment suppressed the hypoxia-induced changes. Additionally, DHA attenuated the expression of hypoxia-activated ERK1/2 in the PASMCs. Conclusions Our observations indicate that DHA may potentially prevent HPH.