Quiescent hematopoietic stem cells (HSCs) are typically dormant, and only a few quiescent HSCs are active. The relationship between “dormant” and “active” HSCs remains unresolved. Here we generated a G0 marker (G0M) mouse line that visualizes quiescent cells. G0M identifies a small population of active HSCs (G0Mlow), which are distinct from dormant HSCs (G0Mhigh), within the conventional quiescent HSC fraction. Single-cell RNA-Seq analyses showed that the gene expression profiles of these populations were nearly identical, yet differed in their Cdk4/6 activity. Furthermore high-throughput small molecule screening revealed that high concentrations of cytoplasmic calcium ([Ca2+]c) were linked to dormancy of HSCs. These findings indicate that G0M separates dormant and active adult HSCs, which are regulated by Cdk4/6 and [Ca2+]c. This G0M mouse line represents a useful resource for investigating physiologically important stem cell subpopulations.