A water-soluble fluorescent aza-BODIPY platform (Wazaby) was prepared and functionalized by a polyazamacrocycle agent and a bioconjugable arm. The resulting fluorescent derivative was characterized and bioconjugated onto a trastuzumab monoclonal antibody as a vector. After bioconjugation, the imaging agent appeared to be stable in serum (>72 h at 37 °C) and specifically labeled HER-2-positive breast tumors slices. The bioconjugate was radiolabeled with [111In] indium and studied in vivo. The developed monomolecular multimodal imaging probe (MOMIP) is water-soluble and chemically and photochemically stable, emits in the near infrared (NIR) region (734 nm in aqueous media), and displays a good quantum yield of fluorescence (around 15%). Single-photon emission-computed tomography and fluorescence imaging have been performed in nude mice bearing HER2-overexpressing HCC1954 human breast cancer xenografts and have evidenced the good tumor targeting of the [111In] In bimodal agent. Finally, the proof of concept of using it as a new tool for fluorescence-guided surgery has been shown.