Conventional dendritic cells (cDCs) are professional antigen-presenting cells that control the adaptive immune response. Their subsets and developmental origins have been intensively investigated but are still not fully understood as their phenotypes, especially in the DC2 lineage and the recently described human DC3s, overlap with monocytes. Here, using LEGENDScreen to profile DC vs. monocyte lineages, we found sustained expression of FLT3 and CD45RB through the whole DC lineage, allowing DCs and their precursors to be distinguished from monocytes. Using fate mapping models, single-cell RNA sequencing and adoptive transfer, we identified a lineage of murine CD16/32⁺CD172a⁺ DC3, distinct from DC2, arising from Ly6C⁺ monocyte-DC progenitors (MDPs) through Lyz2⁺Ly6C⁺CD11c⁻ pro-DC3s, whereas DC2s develop from common DC progenitors (CDPs) through CD7⁺Ly6C⁺CD11c⁺ pre-DC2s. Corresponding DC subsets, developmental stages, and lineages exist in humans. These findings reveal DC3 as a DC lineage phenotypically related to but developmentally different from monocytes and DC2s.