Phase-selective gelation refers to the selective gelation of one phase in an immiscible mixture. Thus far, all such examples have involved a molecular gelator forming nanofibers in (and thus gelling) the oil phase in an oil/water mixture. Here, for the first time, we report the counterpart to the above phenomenon, i.e., selective gelation of the water phase in an oil/water mixture (while leaving the oil undisturbed). This has been a challenging problem because moieties that gel water tend to be either amphiphilic or oil-soluble; thus, if combined with an oil/water mixture, they invariably form an emulsion. Our approach solves this problem by exploiting the tunable self-assembly of laponite (LAP) nanoparticles. Initially, LAP nanoparticles (25 nm disks) are dispersed in water, where they remain unaggregated due to the steric stabilization provided by a triblock copolymer (Pluronic P123) adsorbed on their surface. Thus, the dispersion is initially a low-viscosity sol. When an immiscible oil such as hexadecane is introduced above the sol, the mixture remains biphasic, and both phases remain unaffected. Next, an organic acid such as butanoic acid (BA) is added to the oil. The BA is oil-soluble but also has limited solubility in the water. Over about 30 min, some of the BA enters the water, whereupon it "activates" the self-assembly of LAP particles into a three-dimensional "house-of-cards" network. Ultimately, the water phase is converted into a homogeneous gel with a sufficient yield stress: the aqueous gel holds its weight in the inverted vial while the oil phase remains a thin liquid that can be poured out of the vial. On the whole, the concept advanced here is about activating nanoparticle assembly in water through an adjacent, immiscible phase. This concept could prove useful in conducting certain separations or reactions in the laboratory as well as in enhanced oil recovery.