<p>Aqueous microdroplets have fascinated scientists with unique properties that deviate from bulk solutions. Understanding microdroplet chemistry has been limited by few spectroscopic measurements on single well-defined microdroplets. Here we present a new technique to record an IR spectrum of a single droplet called Single Droplet Displacement Infrared Action Spectroscopy (SiDDIRAS) with potential broad applications in physical, analytical, and environmental chemistry. The droplet was trapped in a linear quadrupole electrodynamic balance. When IR radiation at resonant frequency was introduced, mass loss from IR-induced evaporation caused the droplet to displace upwards. Mapping the displacement as a function of IR frequency yields an IR spectrum of a single microdroplet. As a demonstration, the SiDDIRA spectrum of an ~8 µm droplet containing NaCl and NaN₃ shows that the azide asymmetric stretch blueshifts by 5 cm^-1 relative to bulk with significant peak broadening, evidence that this droplet is supersaturated with salts compared to bulk solutions.</p>