Pulmonary hypertension (PH) remains a life-limiting disease characterized by pulmonary vascular remodelling due to aberrant proliferation and migration of pulmonary artery smooth muscle cells (PASMCs), thus leading to raised pulmonary arterial pressure and right ventricular hypertrophy. Secreted glycoprotein follistatin-like 1 (FSTL1) has been reported to ameliorate tissue remodelling in cardiovascular injuries. However, the role of FSTL1 in deranged pulmonary arteries remains elusive. We found that there were higher serum levels of FSTL1 in patients with PH related to chronic obstructive pulmonary diseases (COPD) and in mice model of hypoxia-induced PH (HPH). Haploinsufficiency of Fstl1 in mice contributed to an exacerbated HPH, as demonstrated by increased right ventricular systolic pressure, pulmonary arterial muscularization and right ventricular hypertrophy index. Conversely, FSTL1 administration attenuated HPH. In cultured human PASMCs, hypoxia-promoted cellular viability, DNA synthesis and migration were suppressed by exogenous FSTL1 but enhanced by small interfering RNA targeting FSTL1. Additionally, FSTL1 inhibited the proliferation and migration of PASMCs via extracellular regulated kinase (ERK) signal pathway. All these findings indicate that FSTL1 imposed a protective modulation on pulmonary vascular remodelling, thereby suggesting its role in the regulation of HPH.