By the end of the Archean Eon, Earth’s surface appears to have formed voluminous felsic crust. However, the nature of this ancient crust remains enigmatic because much of this crust has been strongly altered and lost its geologic context over Earth’s history. Here we use europium systematics recorded by zircon to constrain the minimum melt oxidation state of the Neoarchean granitic rocks. We find that those samples formed at medium to high pressures are oxidized, perhaps as oxidized as those related with porphyry copper deposits in the Phanerozoic. For these oxidized samples, our crystallization modeling places a lower bound on the melt oxygen fugacity at 0-2 log units above the Ni-NiO buffer. The oxidized signatures of these granitic melts are best explained as a consequence of endogenic oxidation due to garnet retention in their sources. Our findings further support that the oxidation state of felsic crustal magmas is connected to crustal thickness, and this connection holds over much of the Earth’s crustal differentiation history. In the late Archean, continuous formation of such oxidized crust may have profoundly influenced volatile output from Earth’s interior and oxidative weathering processes at the surface.