The pluripotency factor LIN28 maintains stem cells in an undifferentiated state by blocking the expression of let-7 microRNAs (miRNAs). LIN28 achieves this by recruiting 3′ terminal uridylyl transferases, which add a terminal oligouridine tail to pre-let-7 that inhibits its processing and promotes miRNA decay. This study identifies the 3′-5′ endonuclease DIS3L2 as the enzyme responsible for promoting pre-let-7 degradation in mouse embryonic stem (ES) cells. DIS3L2 was found to preferentially degrade uridylated pre-let-7 over unmodified pre-let-7 or unrelated miRNAs in vitro. Moreover, siRNA-mediated depletion of DIS3L2 in mouse ES cells resulted in the accumulation of uridylated pre-let-7, whereas knockdown of the exosome 3′-5′ endonucleases EXOSC10 or RRP44 had no effect, confirming that DIS3L2 is the nuclease responsible for carrying out pre-let-7 decay in the LIN28 pathway.