2D heterogeneous oxide nanosheets (NSs) have attracted much attention in various scientific fields owing to their exceptional physicochemical properties. However, the fabrication of 2D oxide NSs with abundant p-n interfaces and large amounts of mesopores is extremely challenging. Here, a facile synthesis of highly porous 2D heterogeneous oxide NSs (e.g., SnO2/CoOx) is suggested through a 2D oxide exfoliation approach combined with a fast galvanic replacement reaction (GRR). The ultrathin (<5 nm) layered CoOx NSs are simply prepared by ion-exchange exfoliation and a subsequent GRR process that induces a rapid phase transition from p-type CoOx to n-type SnO2 metal oxides (<10 min). The controlled GRR process enables the creation of heterogeneous SnO2/CoOx NSs consisting of small SnO2 grain sizes (<10 nm), high porosity, numerous heterojunctions, and sub-10 nm thickness, which are highly advantageous characteristics for chemiresistive sensors. Due to the advantage of these features, the porous SnO2/CoOx NSs exhibit an unparalleled HCHO-sensing performance (R-air/R-gas > 35 @ 5 ppm with a response speed of 9.34 s) with exceptional selectivity compared to that of the state-of-the-art metal oxide-based HCHO gas sensors.