High-molecular-weight hyaluronan, a major component of the extracellular matrix, is anti-oncogenic, whereas low-molecular-weight hyaluronan is pro-oncogenic, though the mechanisms underlying the size-dependent opposite bioactivities of hyaluronan remain uncertain. We show here that treatment with high-molecular-weight hyaluronan stimulates tumor-suppressive Hippo signaling in breast epithelial cells. Mechanistically, clustering of the CD44 extracellular domain by high-molecular-weight hyaluronan leads to recruitment of the polarity-regulating kinase PAR1b by the CD44 intracellular domain, which results in disruption of the Hippo signaling-inhibitory PAR1b-MST complex. Once liberated from PAR1b, MST activates Hippo signaling. Conversely, low-molecular-weight hyaluronan, which is produced by hyaluronidase-mediated degradation of high-molecular-weight hyaluronan, inhibits Hippo signaling by competing with high-molecular-weight hyaluronan for CD44 binding. Triple-negative breast cancers with higher hyaluronidase-2 expression show poorer prognosis than those with lower hyaluronidase-2 expression. Consistently, decreased hyaluronidase-2 is associated with reduced tumorigenicity in a tumor xenograft model. Hence, perturbation of high-molecular-weight hyaluronan-mediated Hippo signaling activation contributes to cancer aggressiveness.