Antimicrobial peptides, a large class of molecules synthetized by various organisms as an innate defense against pathogens are more and more used for their anticancer properties as well. In order to overcome some of their limitations and to enhance their therapeutic efficiency, the use of delivery systems was taken into consideration. In this study we describe an original delivery system for antimicrobial peptides based on its physico-chemical properties, namely the selfassembled polyion complexes (PIC) based on electrostatic interactions of cationic antimicrobial peptide P6 with negatively charged double hydrophilic block copolymers, the poly(ethylene oxide)-poly(acrylic acid) (PEO-PAA) in our study. The drug delivery system was tested on 3D human tumor HCT-116 spheroids. The spheroid evolution and cell viability were monitored at 24 and 48 h after the treatment was applied. Our study demonstrates for the first time the feasibility of forming a polyion complex PIC with an antimicrobial peptide and that this self-assembled organisation provides added value in terms of anti-tumour therapeutic efficacy compared to the free form of the antimicrobial peptide.