Passive fluidic components that are capable of rectifying fluid flow at ultra-low Reynolds Number are critical to the advancement of micro/nanofluidic circuitry for diverse chemical and biological applications, such as sample preparation on chip, molecular diagnostics, point-of-care (POC) testing, and quantitative cell biology platforms. Previously, a wide range of diodic components have been developed to rectify flow in fluidic systems; however, engineering microfluidic diodes that function at the ultra-low Reynolds Number flows (i.e., Re < 0.25) of emerging micro/nanofluidic platforms has remained a considerable challenge. Here we present a microfluidic single-microbead-based diode (SMD) that uses a single suspended microbead as a dynamic resistive element to rectify fluid flow under Re ≤ 0.25 conditions. Simulations of the SMD yielded a theoretical diodicity (Di) of 1.4. Experiments for Re varying from 0.05 to 0.25 revealed average Di's ranging from 1.14±0.01 to 2.51±0.03.