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Background: Pneumocystis jirovecii pneumonia (PJP) is a potentially fatal infection afflicting the immunocompromised population, including solid organ transplant (SOT) recipients. Several risk factors have been described; however, little is known regarding the risk of PJP in SOT recipients with posttransplant lymphoproliferative disorder (PTLD). Methods: We performed a nested case-control study of SOT recipients diagnosed with PJP from 2000 to 2020. PJP was defined as positive microscopy or polymerase chain reaction testing with compatible symptoms and radiographic findings. Control patients were matched 2:1 by year of first transplant, first transplanted organ, transplant center, and sex. Multivariable conditional logistic regression was performed to test associations with PJP and Cox regression analyzed post-PJP outcomes. Results: Sixty-seven PJP cases were matched to 134 controls. The most common transplant was kidney (55.2%). Fourteen patients had a history of PTLD, 12 of whom developed PJP. After adjusting for age, acute rejection, cytomegalovirus infection, PJP prophylaxis, and lymphopenia (lymphocyte count < .5 × 109/L), PTLD was independently associated with PJP (OR 14.0, 95% CI 1.7–114.5; p = .014). Lymphopenia was also a significant association (OR 8.2, 95% CI 3.2–20.7; p < .001). PJP was associated with mortality within 90 days of diagnosis (p < .001), but not after 90 days (p = .317). PJP was also associated with 90-day death-censored renal allograft loss (p = .026). Conclusions: PTLD is independently associated with PJP after adjustment for recognized risk factors. This is likely influenced by PTLD-directed chemotherapy, particularly rituximab-containing regimens. PJP is associated with early mortality, but this effect is not persistent after 90 days. PJP prophylaxis should be considered in SOT recipients with PTLD.