The detection and identification of nanoparticles is of growing interest in atmospheric monitoring, medicine, and semiconductor manufacturing. While elastic light scattering with interferometric detection provides good sensitivity to single particles, active optical components prevent scalability of realistic sizes for deployment in the field or clinic. Here, we report on a simple phase-sensitive nanoparticle detection scheme with no active optical elements. Two measurements are taken simultaneously, allowing the amplitude and phase to be decoupled. We demonstrate the detection of 25 nm Au particles in liquid in Δt ∼ 1 ms with a signal-to-noise ratio of ∼ 37. Such performance makes it possible to detect nanoscale contaminants or larger proteins in real time without the need of artificial labeling.