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Objective: Pneumocystis jirovecii pneumonia (PJP) is a serious opportunistic infection mainly diagnosed in patients with rheumatic conditions. However, PJP in anti-melanoma differentiation-associated gene 5-positive dermatomyositis (MDA5 + DM) patients remains poorly understood. We aimed to investigate the 6-month PJP risk in newly diagnosed MDA5 + DM patients. Methods: A retrospective observational study of 105 inpatients with newly diagnosed MDA5 + DM was conducted at Renji Hospital from January 2018 to November 2019. Demographic information, clinical characteristics, and treatment data were recorded. The primary outcome was PJP incidence within 6 months after a MDA5 + DM diagnosis. Results: The analysis included 105 patients, including 13 patients diagnosed with PJP during the observation period. The median time from the MDA5 + DM diagnosis to PJP was 89 ± 38 days. Compared with the PJP − patients, the PJP + patients had a significantly greater risk of mortality (69.2% vs. 13.0% P < 0.001). Regarding the baseline comorbidities, hypertension (P = 0.013) and cancer (P = 0.02) were more common in the PJP + group. Additionally, a larger proportion of the PJP + patients received prolonged high-dose steroid therapy (≥ 60 mg/day and ≥ 1 month) (P = 0.022) and double or triple immunosuppressant therapy (P = 0.013). The multivariate analysis showed that PJP was independently associated with lymphopenia (ALC < 500 cells/µl) (OR: 5.434, 95% CI: 2.074–55.155; P = 0.012) and the combined use of cyclophosphamide (CTX) and tacrolimus (TAC) (OR: 10.695, 95% CI: 1.440–20.508; P = 0.005). Conclusion: There was a high incidence and mortality in the MDA5 + DM patients with PJP, with patients on combined immunosuppressive treatments, particularly CTX and TAC, being at a higher risk. Prolonged high-dose steroid therapy (≥ 60 mg/day and ≥ 1 month) was another risk factor for PJP. Key Points • There was a high incidence and mortality in the MDA5 + DM patients with PJP. • Most PJP cases occurred within 3 months after the MDA5 + DM diagnosis. • The 6-month infection risk of PJP increased with the administration of multiagent immunosuppression, especially the combination of CTX and TAC. • Prolonged high-dose steroid therapy (≥ 60 mg/day and ≥ 1 month) was another risk factor for PJP. [ABSTRACT FROM AUTHOR]