MicroRNAs (miRNAs) are small, endogenous RNAs that regulate gene expression by binding to complementary messenger RNAs. In animals, miRNAs are generated from hairpin structures in primary transcripts through two sequential cleavages by the RNase III enzymes Drosha and Dicer. However, this study identifies an alternative pathway for miRNA biogenesis involving intronic microRNA precursors (mirtrons) that bypass Drosha processing. These mirtrons, found in *Drosophila melanogaster* and *Caenorhabditis elegans*, mimic pre-miRNA structures and enter the miRNA pathway without Drosha cleavage. The study shows that mirtrons are processed through splicing and debranching, leading to the generation of mature miRNAs. Mirtrons are characterized by their structural similarity to pre-miRNAs and their conservation across species, suggesting important regulatory roles. The abundance of introns similar in size to pre-miRNAs in flies and nematodes likely facilitated the emergence of mirtrons. The study also demonstrates that mirtrons can function as functional miRNAs, repressing gene expression in *Drosophila* cells. Functional assays show that mirtrons, such as miR-1003 and miR-1006, effectively silence target genes, similar to canonical miRNAs. The study further shows that mirtron processing depends on splicing and debranching, with mutations in splicing sites impairing mirtron maturation. The debranching enzyme is essential for mirtron processing, as its knockdown reduces mirtron miRNA levels. Additionally, the study reveals that mirtrons are processed through a pathway distinct from the canonical miRNA pathway, which relies on Drosha. The findings suggest that mirtrons could have provided an early pathway for miRNA emergence before the evolution of Drosha. The study also highlights the evolutionary conservation of mirtrons, with some showing higher conservation than other small introns. The presence of mirtrons in species with appropriately sized introns supports their role in miRNA biogenesis. The study provides insights into the diverse mechanisms of miRNA biogenesis and the potential for intronic sequences to function as miRNA precursors.MicroRNAs (miRNAs) are small, endogenous RNAs that regulate gene expression by binding to complementary messenger RNAs. In animals, miRNAs are generated from hairpin structures in primary transcripts through two sequential cleavages by the RNase III enzymes Drosha and Dicer. However, this study identifies an alternative pathway for miRNA biogenesis involving intronic microRNA precursors (mirtrons) that bypass Drosha processing. These mirtrons, found in *Drosophila melanogaster* and *Caenorhabditis elegans*, mimic pre-miRNA structures and enter the miRNA pathway without Drosha cleavage. The study shows that mirtrons are processed through splicing and debranching, leading to the generation of mature miRNAs. Mirtrons are characterized by their structural similarity to pre-miRNAs and their conservation across species, suggesting important regulatory roles. The abundance of introns similar in size to pre-miRNAs in flies and nematodes likely facilitated the emergence of mirtrons. The study also demonstrates that mirtrons can function as functional miRNAs, repressing gene expression in *Drosophila* cells. Functional assays show that mirtrons, such as miR-1003 and miR-1006, effectively silence target genes, similar to canonical miRNAs. The study further shows that mirtron processing depends on splicing and debranching, with mutations in splicing sites impairing mirtron maturation. The debranching enzyme is essential for mirtron processing, as its knockdown reduces mirtron miRNA levels. Additionally, the study reveals that mirtrons are processed through a pathway distinct from the canonical miRNA pathway, which relies on Drosha. The findings suggest that mirtrons could have provided an early pathway for miRNA emergence before the evolution of Drosha. The study also highlights the evolutionary conservation of mirtrons, with some showing higher conservation than other small introns. The presence of mirtrons in species with appropriately sized introns supports their role in miRNA biogenesis. The study provides insights into the diverse mechanisms of miRNA biogenesis and the potential for intronic sequences to function as miRNA precursors.