SERINC5 is a host cell protein that inhibits the infectivity of HIV-1 by a novel and poorly understood mechanism. Here, we provide evidence that the SERINC5 protein alters the conformation of the HIV-1 Env proteins and that this action is correlated with SERINC5’s ability to inhibit HIV-1 infectivity. Defining the specific effects of SERINC5 on the HIV-1 glycoprotein conformation may be useful for designing new antiviral strategies targeting Env.