α-Emission radiotherapeutics has potential to be one of most effective cancer therapeutics. Herein, we report a facile synthesis of an At-labeled immunoconjugate for use as an α-emission molecular targeting therapy. We synthesized a tetrazine probe modified with -decaborate(2-), a prosthetic group that forms a bioavailable stable complex with At. Our one-pot three-component double-click labeling method was used to attach decaborate to trastuzumab (anti-HER2 antibody) using decaborate-tetrazine and TCO-aldehyde probes without reducing the antibody binding affinity. Labeling the decaborate-attached trastuzumab with At produced in the cyclotron at the RIKEN Nishina Center, at which highly radioactive At can be produced, readily furnished the At-labeled trastuzumab with a maximum specific activity of 15 MBq μg and retention of the native binding affinity. Intratumor injection of the At-labeled trastuzumab in BALB/c nude mice implanted with HER2-expressing epidermoid cancer cells yielded efficient accumulation at the targeted tumor site as well as effective suppression of tumor growth.