This paper provides evidence for electrical 1/f noise as the dominant source of excess noise in piezoresistive microelectromechanical systems (MEMS) microphones. In piezoresistors, the fundamental noise sources may be divided into frequency independent thermal noise and frequency dependent 1/f excess noise dominating at low frequencies. Noise power spectra are presented for both commercial and research-prototype MEMS piezoresistive microphones as a function of applied voltage bias for both free and blocked membranes. The contributions of various mechanical and electrical noise sources are compared using a lumped noise equivalent circuit of the piezoresistive microphone. The bias dependence and membrane independence of the output noise indicate that the primary source of the excess noise is electrical in origin.