NiFe oxyhydroxide (NiFe−OH) has shown promising electrocatalytic oxygen evolution reaction (OER) activity. Here we suggest that the performance of NiFe−OH can be further enhanced by constructing pn junction. Using MnCo carbonate hydroxide (MnCo−CH)@NiFe−OH pn junction as a demonstration, we show that upon the construction of pn junction, the electrons flow from n-type NiFe−OH to MnCo−CH, which consequently generates a positively charged region on NiFe−OH. The density function theory calculation reveals that such an electronic property change results in an improved OER energetics. As a result, the MnCo−CH@NiFe−OH pn junction shows significantly enhanced OER performance that is ∼10 and ∼500 times that of NiFe−OH and MnCo−CH (in terms of the OER currents at the overpotential of 270 mV), respectively. Moreover, the pn junction also shows a greatly boosted hydrogen evolution reaction (HER) and therefore the overall water electrolysis activity that outperforms the Pt/C||RuO2 catalysts.