The GRPR-antagonist-based radioligands [Ga/In/Lu]NeoBOMB1 have shown excellent theragnostic profiles in preclinical prostate cancer models, while [Ga]NeoBOMB1 effectively visualized prostate cancer lesions in patients. We were further interested to explore the theragnostic potential of NeoBOMB1 in GRPR-positive mammary carcinoma, by first studying [Ga]NeoBOMB1 in breast cancer models; Methods: We investigated the profile of [Ga]NeoBOMB1, a [Ga]NeoBOMB1 surrogate, in GRPR-expressing T-47D cells and animal models; : NeoBOMB1 (ICs of 2.2 ± 0.2 nM) and [Ga]NeoBOMB1 (ICs of 2.5 ± 0.2 nM) exhibited high affinity for the GRPR. At 37 °C [Ga]NeoBOMB1 strongly bound to the T-47D cell-membrane (45.8 ± 0.4% at 2 h), internalizing poorly, as was expected for a radioantagonist. [Ga]NeoBOMB1 was detected >90% intact in peripheral mouse blood at 30 min pi. In mice bearing T-47D xenografts, [Ga]NeoBOMB1 specifically localized in the tumor (8.68 ± 2.9% ID/g vs. 0.6 ± 0.1% ID/g during GRPR-blockade at 4 h pi). The unfavorably high pancreatic uptake could be considerably reduced (206.29 ± 17.35% ID/g to 42.46 ± 1.31% ID/g at 4 h pi) by increasing the NeoBOMB1 dose from 10 pmol to 200 pmol, whereas tumor uptake remained unaffected. Notably, tumor values did not decline from 1 to 24 h pi; Conclusions: [Ga]NeoBOMB1 can successfully target GRPR-positive breast cancer in animals with excellent prospects for clinical translation.