Human cancers often exhibit attenuated microRNA (miRNA) biogenesis and global underexpression of miRNAs; thus, targeting the miRNA biogenesis pathway represents a novel strategy for cancer therapy. Here, we report that miR-26a enhances miRNA biogenesis, which acts as a common mechanism partially accounting for miR-26a function in diverse cancers including melanoma, prostate and liver cancer. miR-26a was broadly reduced in multiple cancers, and overexpression of miR-26a significantly suppressed tumor growth and metastasis both in vitro and in vivo, including melanoma, prostate and liver cancers. Notably, miR-26a overexpression was accompanied by global upregulation of miRNAs, especially let-7, and let-7 expression was concordant with miR-26a expression in cancer cell lines, xenograft tumors and normal human tissues, underscoring their biological relevance. We showed that miR-26a directly targeted Lin28B and Zcchc11—two critical repressors of let-7 maturation. Furthermore, we have demonstrated that Zcchc11 promoted tumor growth and metastasis, and it was prominently overexpressed in human cancers. Our findings thus provide a novel mechanism by which a miRNA acts as a modulator of miRNA biogenesis. These results also define a role of the miR-26a and Zcchc11 in tumorigenesis and metastasis and have implications to develop new strategies for cancer therapy.