Although achieving optical gain using aqueous solutions of colloidal nanocrystals as a gain medium is exceptionally beneficial for bio-optoelectronic applications, the realization of optical gain in an aqueous medium using solution-processed nanocrystals has been extremely challenging because of the need for surface modification to make nanocrystals water dispersible while still maintaining their gain. Here, we present the achievement of optical gain in an aqueous medium using an advanced architecture of CdSe/CdS@CdxZn1−xS core/crown@gradient-alloyed shell colloidal quantum wells (CQWs) with an ultralow threshold of ∼3.4 μJ cm−2 and an ultralong gain lifetime of ∼2.6 ns. This demonstration of optical gain in an aqueous medium is a result of the carefully heterostructured CQWs having large absorption cross-section and gain cross-section in addition to inherently slow Auger recombination in these CQWs. Furthermore, we show low-threshold in-water amplified spontaneous emission (ASE) from these aqueous CQWs with a threshold of 120 μJ cm−2. In addition, we demonstrate a whispering gallery mode laser with a low threshold of ∼30 μJ cm−2 obtained by incorporating films of CQWs by exploiting layer-by-layer approach on a fiber. The observation of low-threshold optical gain with ultralong gain lifetime presents a significant step toward the realization of advanced optofluidic colloidal lasers and their continuous-wave pumping.