In the present work, we show for the first time that the conversion of aldehydes to alcohols can be achieved using "unprotected" iridium transfer hydrogenation catalysts inside living cells. The reactions were observed in real time by confocal fluorescence microscopy using a Bodipy fluorogenic substrate. We propose that the reduced cofactor nicotinamide adenine dinucleotide (NADH) is a possible hydride source inside the cell based on studies using pyruvate as a cellular redox modulator. We expect that this biocompatible reductive chemistry will be broadly useful to practitioners working at the interface of chemistry and the life sciences.