Background: The effect of trastuzumab therapy on left atrial (LA) function remains largely unknown. Our aim was to assess the changes in LA strain parameters longitudinally in patients treated with trastuzumab. Methods: We retrospectively studied 170 patients with stage I-IV HER2+ breast cancer. All patients had baseline echocardiograms and repeat echocardiograms at 3 months and after 1 year. We measured LA strain at all three time points. Changes in LA strain and strain rate (sr) parameters were evaluated using repeated-measures mixed-effects models. The cohort was stratified according to development of cancer therapeutics-related cardiac dysfunction (CTRCD) during follow-up. Results: The mean age was 52.7 ± 13.8 years, 25.3% had hypertension and 16.0% had metastatic disease. Multiple LA strain parameters (predicted delta value, [95%CI]) showed statistically significant declines in patients who developed CTRCD from baseline to the 3-month follow-up after multivariable adjustment; LA reservoir strain (LAε_res): −4.7%; [−8.1% to −1.3%], p =.007; LA conduit strain (LAε_con): −2.8%; [−5.3% to −.4%], p =.021); and LAε_res sr: −.2/s; [−.3/s to −.09/s], p <.001). In patients who did not develop CTRCD, LA strain parameters declined significantly but to a smaller degree than in the CTRCD group (LAε_res: −1.7%; [−3.1% to −.3%], p =.020, LAε_con: −2.2%; [−3.3% to −1.1%], p <.001, and LA booster pump strain : −2.4%; [−3.5% to −1.4%], p <.001). LA strain rates did not decline significantly in the non-CTRCD group. Conclusion: Trastuzumab treatment was associated with declines in LA strain parameters in patients with breast cancer. The largest declines were observed in patients who developed CTRCD during treatment.