Nonmuscle myosin-IIA (NMHC-IIA) heavy chain phosphorylation has gained recognition as an important feature of myosin-II regulation. In previous work, we showed that phosphorylation on S1943 promotes myosin-IIA filament disassembly in vitro and enhances EGF-stimulated lamellipod extension of breast tumor cells. However, the contribution of NMHC-IIA S1943 phosphorylation to the modulation of invasive cellular behavior and metastasis has not been examined. Stable expression of phosphomimetic (S1943E) or non-phosphorylatable (S1943A) NMHC-IIA in breast cancer cells revealed that S1943 phosphorylation enhances invadopodia function, and is critical for matrix degradation in vitro and experimental metastasis in vivo. These studies demonstrate a novel link between NMHC-IIA S1943 phosphorylation, the regulation of extracellular matrix degradation and tumor cell invasion and metastasis. •pS1943 NMHC-IIA is detected in orthotopic tumors and metastatic lesions.•S1943E NMHC-IIA drives gelatin degradation and MMP9 secretion.•S1943E NMHC-IIA S1943E increases experimental metastasis in vivo.•NMHC-IIA S1943 phosphorylation regulates breast cancer invasion and metastasis.