Background: Ado-trastuzumab emtansine (T-DM1) is the standard of care for patients with advanced HER2 positive breast cancer (BC) patients who progress on the first line anti-HER2 therapy or relapse within 6 months of adjuvant trastuzumab. There is a lack of data regarding the safety and efficacy of T-DM1 in the Indian population. Methods: This retrospective study aims to evaluate the efficacy and toxicity of T-DM1 in advanced HER2 positive female BC patients, registered at All India Institute of Medical Sciences, (AIIMS) New Delhi, India, between the years 2015-2021. The response was evaluated by Response evaluated Criteria in Solid Tumors(RECIST 1.1)guidelines. Progression-free survival (PFS) was calculated from the beginning of treatment with T-DM1 until disease progression or death from any cause. Overall survival (OS) was calculated from the beginning of treatment with T-DM1 until death from any cause. Results: This study included eighty patients with a median age of 49 years (27-73). Forty-four (55%) were premenopausal and hormone receptor positivity (either ER or PR) was found in 36 (45%) cases. The ECOG performance status was 0-1in 80% of patients. De- novo metastatic disease was found in 38(47.5%) cases whereas the remaining 42 (52.5%) patients were diagnosed with early or locally advanced disease and later become metastatic. The sites of metastasis were lungs in 26 (32.5%), brain in 23 (28.7%) liver in 23 (28.7%), bones in 18 patients (22.5%), lymph nodes in 15(18.7%), cutaneous and other rare sites in 8(10%) cases. All patients had received trastuzumab in the prior line of treatment and 18 (22.5%) patients had also received pertuzumab along with trastuzumab. The median number of T-DM1 cycles given was 9 (range 3-35). The overall response rate was 80%, 8(10%) achieved a complete response, 36 (45%) had a partial response, and 12 (15%) patients had stable disease. The Grade 3/4 toxicities were observed in 20(25%) cases and these were: thrombocytopenia 16 (20%), fatigue 10 (12.5%), transaminitis 10(12.5%), anemia 2 (2.5%), and peripheral neuropathy in 2 (2.5%) cases. Dose delay, interruption, and modification were observed in 25 (31.2%) cases. Within the median follow-up period of 18 months, the median PFS was 10 months and OS was 27 months. Hormone status (ER/PR) and prior pertuzumab therapy did not influence the outcome. Conclusions: This is the first study from the Indian subcontinent on T-DM1. It is safe and effective in our population and has shown comparable outcomes with the available literature however, higher rates of thrombocytopenia were observed.