RNase III Drosha initiates microRNA (miRNA) maturation by cleaving a primary miRNA transcript and releasing a pre-miRNA with a 2 nt 3′ overhang. Dicer recognizes the 2 nt 3′ overhang structure to selectively process pre-miRNAs. Here, we find that, unlike prototypic pre-miRNAs (group I), group II pre-miRNAs acquire a shorter (1 nt) 3′ overhang from Drosha processing and therefore require a 3′-end mono-uridylation for Dicer processing. The majority of let-7 and miR-105 belong to group II. We identify TUT7/ZCCHC6, TUT4/ZCCHC11, and TUT2/PAPD4/GLD2 as the terminal uridylyl transferases responsible for pre-miRNA mono-uridylation. The TUTs act specifically on dsRNAs with a 1 nt 3′ overhang, thereby creating a 2 nt 3′ overhang. Depletion of TUTs reduces let-7 levels and disrupts let-7 function. Although the let-7 suppressor, Lin28, induces inhibitory oligo-uridylation in embryonic stem cells, mono-uridylation occurs in somatic cells lacking Lin28 to promote let-7 biogenesis. Our study reveals functional duality of uridylation and introduces TUT7/4/2 as components of the miRNA biogenesis pathway. Display omitted ▸ Unlike canonical group I miRNA precursors, group II members have a 1 nt 3′ overhang ▸ Mono-uridylation of group II pre-let-7 promotes Dicer processing ▸ TUT7, TUT4, and TUT2 catalyze pre-let-7 mono-uridylation in cells lacking Lin28 ▸ Lin28 induces oligo-uridylation to inhibit let-7 processing Group II precursors require mono-uridylation by TUTases to create the overhang of 2 nt required for Dicer processing. In the presence of Lin28, let-7 is preferably oligo-uridylated by TUT4 promoting its degradation, revealing the functional duality of uridylation.