Electron transfer (ET) is widely used for driving the processes that underlie the chemistry of life. However, our abilities to probe electron transfer mechanisms in proteins and design redox enzymes are limited, due to the lack of methods to site-specifically insert electron acceptors into proteins in vivo. Here we describe the synthesis and genetic incorporation of 4-fluoro-3-nitrophenylalanine (FNO₂Phe), which has similar reduction potentials to NAD(P)H and ferredoxin, the most important biological reductants. Through the genetic incorporation of FNO₂Phe into green fluorescent protein (GFP) and femtosecond transient absorption measurement, we show that photoinduced electron transfer (PET) from the GFP chromophore to FNO₂Phe occurs very fast (within 11 ps), which is comparable to that of the first electron transfer step in photosystem I, from P700* to A₀. This genetically encoded, low-reduction potential unnatural amino acid (UAA) can significantly improve our ability to investigate electron transfer mechanisms in complex reductases and facilitate the design of miniature proteins that mimic their functions.