A 35-year-old woman with primary Sjögren's syndrome (pSS) developed fever and chest pain during pregnancy. When the dose of prednisolone was reduced, she experienced chest pain with elevated CRP and D-dimer, resulting in admission to our hospital with marked cardiomegaly and pleural effusion. Because there was no evidence of other autoimmune disorders or infection, oral prednisolone was increased to 30 mg daily with heparin, and hypercoagulopathy was carefully monitored. The patient's condition improved rapidly, and she delivered a healthy baby. This is the first case to support the beneficial effect of prednisolone in pericarditis with pSS, and illustrates its safety during pregnancy.