Fucoidan is a fucose-rich polysaccharide that has gained attention for its various anticancer properties. However, the effect and underlying mechanism of fucoidan on triple-negative breast cancer (TNBC) are still unknown. Herein, we investigated the anticancer potential of fucoidan from Laminaria japonica. We found that fucoidan showed modest antiproliferative activity against TNBC cells, while it effectively reduced migratory and invasive capacities. Mechanistically, fucoidan suppressed activation of MAPK and PI3K followed by inhibition of AP-1 and NF-κB signaling in TNBC. Additionally, fucoidan downregulated expressions of proangiogenic factors in TNBC cells, and fucoidan blocked tumor-elicited tube formation by human umbilical vascular endothelial cells (HUVECs). We also observed that fucoidan blocked tumor adhesion and invasion towards HUVECs. Surprisingly, fucoidan robustly suppressed tube formation on HUVECs. Moreover, fucoidan inhibited in vivo angiogenesis and micrometastasis in a transgenic zebrafish model. Together, L. japonica fucoidan exhibits potent antitumor effects by its attenuation of invasiveness and proangiogenesis in TNBC. •Fucoidan from Laminaria japonica exerts antitumor effects.•Fucoidan suppresses proangiogenesis and micrometastasis in TNBC.•Fucoidan blocks MAPK and PI3K signaling in TNBC.