This study investigates the development of innovative polyelectrolyte complexes (PECs) based on polysaccharides, carrageenan (CRG) and cyanocobalamin-chitosan (CSB12), for the enhanced delivery of colistin (CT), a last-resort antibiotic used to treat multidrug-resistant Gram-negative bacterial infections. Two novel systems were engineered: CRG-CT and CRG-CT-CSB12 PECs. The CRG-CT PECs displayed a hydrodynamic diameter of 220 nm and a ζ-potential of −35.6 mV, whereas the CRG-CT-CSB12 PECs were smaller (140 nm) and exhibited a positive charge (ζ-potential of +30.1 mV). For CRG, the binding constants for CT interactions were reported for the first time, demonstrating a twofold increase in strength compared to those of hyaluronic acid. This enhanced binding affinity correlates with prolonged drug release and a more pronounced reduction in nephro- and neurotoxicity relative to previously established systems. Both systems demonstrated sustained release of CT, with only 23 % of the drug released over 24 h, and a two fold reduction in the minimum inhibitory concentration against Pseudomonas aeruginosa compared to free CT. Notably, the PECs showed no nephro- or neurotoxicity in human cell line models (HEK 293 and T-98G). The CRG-CT-CSB12 PECs also exhibited superior intestinal permeability (Papp = 7.4 × 10−6 cm/s) in Caco-2 cell monolayers, indicating their suitability for oral administration. In vivo studies in a rodent model confirmed the stability of CRG-CT PECs in the bloodstream and their lack of acute toxicity at therapeutic doses. These findings highlight the potential of CRG-CT and CRG-CT-CSB12 PECs as effective, and innovative nanoantibiotics for combating multidrug-resistant infections, with particular promise for oral delivery. © 2025 Elsevier B.V., All rights reserved.