Background: The U.S. Food and Drug Administration (FDA) and European Medicines Agency (EMA) defines a biosimilar as a biological molecule product that is highly similar to and has no clinically meaningful differences from an existing approved reference product. The trastuzumab biosimilar was the first oncology biosimilar approved in Brazil for HER2-positive breast cancer (BC) and advanced gastric cancer treatment. This study aimed to assess the effectiveness and safety of trastuzumab biosimilar (named as Zedora in Brazil) in the adjuvant setting among HER2-positive BC patients treated in Brazil. Methods: This is a national, multicentric, observational, and prospective real-world study. Patients with early-stage HER2-positive BC who received at least one dose of trastuzumab biosimilar as adjuvant therapy were eligible. At the present date, 126 of 170 planned patients were recruited. Here, we present the preliminary safety data of the first 42 patients recruited in the study. The safety endpoint is the occurrence of adverse events (AEs), and the safety population included all patients who received at least one dose of trastuzumab biosimilar. We analyzed the AEs collected from the signature of the consent form until the end of adjuvant treatment. Results: Baseline patient characteristics among the 42 evaluated patients included a median age of 48.5 years, with 78.6% of them having a node-negative disease. Most patients had hormone-receptor-positive tumors (71.4% estrogen-receptor-positive and 57.1% progesterone-receptor-positive). The number of patients undergoing conservative surgery (59.5%) was higher than mastectomy (40.5%). The most frequent histological BC subtype was invasive ductal carcinoma (97.6%). Neoadjuvant therapy included the trastuzumab biosimilar administration (25%) and the dual anti-HER2 block (trastuzumab biosimilar + pertuzumab) (68.8%). Only 6.3% of patients received the reference trastuzumab in neoadjuvant therapy. In total, 25/42 (59,5%) patients had at least one AE, two of them were serious AEs (4.8%). The incidence of grade 3 or 4 AEs was 35.7%, and no death occurred. The table lists the most reported AEs according to SOC (System Organ Class). Conclusions: The nature and severity of AEs observed were consistent with the known safety profile of trastuzumab. Clinical trial information: NCT03892655.