Background: Amplification and dimerization of HER2 promotes growth and survival of malignant cells. Tumor responses to available therapeutic agents targeting HER2 are variable. Re-activation of the potent HER2:HER3 signaling dimer by up-regulation of the HER3 ligand heregulin (HRG) has recently been identified as an important resistance mechanism. Treatment of patients with tumors expressing HER2 could be improved with agents that specifically target and potently inhibit the HER2:HER3 heterodimer. Methods: Inhibition of HRG mediated cell growth and invasiveness was assessed by proliferation inhibition and high content imaging. ADCC activity was measured by standard PBMC assays and reporter cell lines. The trastuzumab-resistant, HER2 amplified JIMT-1 xenograft model was used to assess in vivo activity. Activation status of HER dimers was determined by specific phosphorylation readouts. Results: MCLA-128 is a human common light chain bispecific antibody targeting HER2 and HER3 selected from > 500 bispecific antibody candidates. MCLA-128 inhibits HER2:HER3 heterodimer phosphorylation. This molecular phenotype correlated with potent inhibition of HRG driven growth and invasiveness in BT474, SKBR3 and N87 cell lines [IC₅₀ 100 – 200 pM]. In contrast, HER3 antibodies, trastuzumab alone or combined with either pertuzumab or HER3 antibodies could not inhibit growth under these conditions even at concentrations two orders of magnitude higher. The ADCC activity of MCLA-128, a low fucosylated IgG1, was equivalent to trastuzumab when targeting HER2^hi cell lines but significantly superior when targeting HER2^lo cell lines and when low affinity FcgRIII effector cells were used. In vivo, weekly dosing of MCLA-128 at 25 mg/kg reduced tumor burden significantly compared to lapatinib or trastuzumab + pertuzumab treatment groups. Conclusions: MCLA-128 specifically and potently inhibits ligand dependent HER2:HER3 signaling resulting in suppression of tumor growth in vitro and in vivo. The unique functional profile of this novel full-length bispecific antibody, including its strong ADCC activity, supports further development and clinical evaluation in patients with HER2 positive tumors.