Background: We reported the results of an Italian large retrospective analysis that evaluated the effectiveness and safety of T-DM1 in ‘field-practice’ breast cancer patients. We performed a sub-analysis to investigate the clinical activity of T-DM1 in patients with brain metastases (BMs). Methods: The records of 87 adult women with HER2-positive breast cancer and BMs treated with T-DM1 were reviewed. Their clinical outcomes were compared with those of 216 patients without central nervous system (CNS) involvement. Results: Response to T-DM1 treatment in BMs was available for 53 patients in the BM group (60.9%): two patients reported a complete response (3.8%), 11 patients obtained partial response (20.7%; overall response rate: 24.5%), 16 patients had a stable disease (30.1%). Regarding extracranial disease, a total of 77 and 191 patients were evaluable for response in BM group and non-BM group, respectively. The overall response rate was 35.1% in the BM group and 38.3% in the non-BM group; disease control rate was 53.3% and 66.6%, respectively. At a median follow-up of 16 months (range: 1–55), median cumulative progression-free survival (PFS) was 7 months (95% CI: 5.4–8.6) in the BM group and 8 months (95% CI: 5.7–10.3) in the non-BM group. In the second-line setting, PFS was 5 (95% CI: 3.1–6.9) versus 11 (95% CI: 7.1–14.9) months (p = 0.01). Overall survival was 14 months (95% CI: 12.2–15.8) in the BM group and 32 months (95% CI: 24.4–39.6) in the non-BM group (p <0.0001). Conclusions: T-DM1 is active in breast cancer patients with BMs.