Saw Zein (Burmese: စောဇိတ်, Burmese pronunciation: [sɔ́ zeiʔ]; also known as Saw Zeik and Binnya Ran De; 1303–1330) was king of Martaban from 1323 to 1330. He inherited a newly independent kingdom from his elder brother Saw O but spent much of his reign putting down rebellions. Although he regained the Mon-speaking provinces of Lower Burma, he could not recover the Tenasserim coast from Martaban's former overlord Sukhothai.

Zein was assassinated in 1330 in a coup organized by Zein Pun, one of his senior commanders. Zein Pun seized the throne only to be killed a week later.

Chronicles provide little information about his early life. Saw Zein was born on 19 May 1303 to Princess Hnin U Yaing and Gov. Min Bala of Myaungmya. He had two other full brothers, and at least one half brother. He was presumably brought up in Myaungmya, a key port in the Irrawaddy delta, where his father was governor. His whereabouts during the reign of his eldest brother King Saw O (r. 1311–1323) is unknown except that Zein was married to his first cousin Princess Sanda Min Hla, and had three children by 1323.

The prince was suddenly thrust into the spotlight in September 1323 when the 39-year-old O died. With O's eldest son, Saw E, still just about 10 years old, the 19-year-old Zein ascended the Martaban throne. At his accession, he made Sanda Min Hla his chief queen but also raised his sister-in-law May Hnin Htapi, who was a Siamese princess, as a principal queen.

At his accession, Zein inherited a fully independent kingdom. O had broken with his nominal overlord Sukhothai since 1319, and taken Lamphun and the Tenasserim coast by 1321. However, in the tradition of the prevailing Southeast Asian administrative model, every new high king had to establish his authority with the vassals all over again. The 19-year-old king garnered no initial support, and the vassal rulers in the Pegu province and delta promptly revolted.

Zein had to rebuild the kingdom all anew. His first target was the Pegu province (present-day Yangon Region and southern Bago Region). He left the capital Martaban (Mottama) with a strong garrison, and marched north with the army. The inaugural campaign was successful; he took Pegu (Bago), Dagon (modern Central Yangon), Dala (modern Dala-Twante) and Watanaw.

His next target was the Irrawaddy delta province, which remained in revolt. He did not return to Martaban, and instead affixed a temporary capital north of Pegu. He went on elephant hunting trips to gather more war elephants for the next campaign. With the renewed force, Zein then invaded the delta, and got the lords of Myaungmya and Bassein (Pathein) to submit.

His job was far from over. While Zein was campaigning in Lower Burma, his nominal vassals on the Tenasserim coast also revolted. Zein sent a force led by Gen. Bon-Yin to retake the coast. The army regained Tavoy but was driven back at Tenasserim (Taninthayi). He sued for a truce with Sukhothai. The king of Sukhothai treated him like a vassal, giving him the title Binnya Ran De (ဗညား ရံဒယ်, Burmese pronunciation: [bəɲá jàɴ dɛ̀]).