MicroRNA deregulation is frequent in human colorectal cancers (CRCs), but little is known as to whether it represents a bystander event or actually drives tumor progression in vivo. We show that miR-135b overexpression is triggered in mice and humans by APC loss, PTEN/PI3K pathway deregulation, and SRC overexpression and promotes tumor transformation and progression. We show that miR-135b upregulation is common in sporadic and inflammatory bowel disease-associated human CRCs and correlates with tumor stage and poor clinical outcome. Inhibition of miR-135b in CRC mouse models reduces tumor growth by controlling genes involved in proliferation, invasion, and apoptosis. We identify miR-135b as a key downsteam effector of oncogenic pathways and a potential target for CRC treatment. •miR-135b is overexpressed in mouse and human colorectal cancer•miR-135b overexpression is associated with poor clinical outcome•miR-135b activation is triggered by oncogenic pathways in colorectal cancer•miR-135b represents a therapeutic target for colorectal cancer Valeri et al. identify miR-135b as a key oncogenic pathway effector involved in transformation and colorectal cancer (CRC) progression. Upregulation of miR-135b in human CRCs correlates with poor clinical outcome. miR-135b targets several tumor suppressor genes and is a potential target for CRC therapy.