Soft (nano)colloids are increasingly used in medical applications due to the versatile options they offer in terms of e.g. tunable chemical composition, adaptable physical properties and (bio)functionalization perspectives. Obtaining a clear understanding of the nature of the interaction forces that such particles experience with neighboring charged (bio)surfaces is a mandatory prerequisite to draw a comprehensive and mechanistic picture of their stability and reactivity and to further optimize their current functionalities. In this study, adopting an original strategy for nanoparticle attachment to atomic force microscopy (AFM) tips, we demonstrate that the sign of electrostatic forces between carboxylate-terminated poly(amidoamine) nanodendrimers (∼9 nm in diameter) and planar cysteamine-coated gold surfaces can be tailored under fixed pH conditions upon the sole variation of the monovalent salt concentration in solution. The origin of this unconventional electrostatic force reversal is deciphered upon confrontation between AFM force measurements and mean-field force evaluation performed beyond the Derjaguin approximation by integrating the dendrimer and cysteamine electrostatic properties derived independently from electrokinetic measurements. It is shown that the electrostatic force reversal (i) originates from the zwitterionic character of the nanodendrimer-solution interphase, and (ii) becomes operational under the strict condition that the sub-nanometric separation distance between peripheral carboxylate groups and intraparticulate amines is of the order of the characteristic electric Debye layer thickness. The possibility to mediate - via suitable adjustment of monovalent salt content in solution - both the magnitude and sign of the electrostatic forces acting on soft interfaces with zwitterionic functionality paves the way for the design of innovative strategies to control the stability of nanoparticles against aggregation, and to modulate their adhesion onto inorganic surfaces or living organisms.