Abstract Background The therapeutic efficacy of cytotoxic anticancer drugs has been reported to be enhanced after immune checkpoint inhibitors (ICI) in non–small cell lung cancer; however, it is unclear whether the same is applicable for small cell lung cancer (SCLC). We evaluated the efficacy of second‐line amrubicin (AMR) following first‐line platinum‐based chemotherapy and ICI combination therapy (chemo‐ICI) in SCLC. Patients and Methods We retrospectively enrolled consecutive patients with SCLC treated with AMR as a second‐line following chemo‐ICI as first‐line between July 2019 and April 2021 from 16 institutions throughout Japan. We investigated the therapeutic effectiveness, safety, and efficacy‐enhancing variables of AMR. Results Overall, 89 patients treated with AMR after first‐line chemo‐ICI were analyzed. The overall response rate (ORR) was 29.2% (95% confidence intervals [CI], 20.1–39.8) and median PFS (m PFS) was 2.99 months (95% CI, 2.27–3.65). Patients who relapsed more than 90 days after receiving first‐line platinum combination therapy (sensitive relapse) exhibited greater ORR (58.3% vs. 24.7%, p = 0.035) and m PFS (5.03 vs. 2.56 months, p = 0.019) than patients who relapsed in