Nonconventional biomacromolecular luminogens have attracted extensive interest due to their fundamental importance and potential applications in diverse areas. To explore novel luminogens and, moreover, to gain deeper insights into their emission mechanism, we study the emission behaviors of sodium alginate (SA), a natural anionic polysaccharide composed of mannuronic (M) and guluronic acids (G). We find that the luminescence from aqueous SA solutions exhibits distinct concentration enhanced emission and aggregation induced emission (AIE) characteristics. Meanwhile, the ratio of M/G also matters. Rheological measurements reveal the distinct regimes of the solutions, which are consistent with the observed emission, indicative of strong association between the chain entanglement and emission. Moreover, we observe persistent room temperature phosphorescence (RTP) in the amorphous SA solids, which is a rare case even in pure organic aromatic luminogens. Such unique emission can be remarkably enhanced via coordination with Ca2+ ions. These emission behaviors can be well rationalized by the clustering-triggered emission (CTE) mechanism. Namely, the emission is caused by the electron cloud overlap due to the clustering of oxygen atoms and carboxylate units, together with conformation rigidification. Owing to its biocompatibility, intrinsic emission, and, moreover, persistent RTP, SA shows great potential for anticounterfeiting, encryption, intracellular imaging, and so on.