Yamada Nagamasa (Japanese: 山田 長政; 1590–1630) or Okya Senaphimuk (Thai: ออกญาเสนาภิมุข) was a Japanese adventurer who gained considerable influence in the Ayutthaya Kingdom at the beginning of the 17th century and became the governor of Nakhon Si Thammarat, which is on the Malay Peninsula in present-day Southern Thailand.

From 1617 until his death in 1630, Yamada Nagamasa was head of the Thai village referred to as Ban Yipun ('Japanese village') in the Thai language. This village was within the city of Ayutthaya (the capital city of the Ayutthaya Kingdom). Ban Yipun was home to roughly 1,000 Japanese citizens and was headed by a Japanese chief who was nominated by Ayutthayan authorities. Its inhabitants were a combination of traders, Christian converts who had fled their home country following the persecutions of Toyotomi Hideyoshi and Tokugawa Ieyasu and rōnin (unemployed former samurai) who had been on the losing side at the Battle of Sekigahara (1600) or the Siege of Osaka (1614–15). The Christian community seems to have been in the hundreds, as described by Padre António Francisco Cardim, who recounted having administered sacraments to around 400 Japanese Christians in 1627 in the city of Ayutthaya.

Yamada Nagamasa was born in Numazu, Shizuoka in 1590. He is said to have been a palanquin bearer for the lord of Numazu. He became involved in Japanese trade activities with Southeast Asia during the period of the red seal ships and settled in the Ayutthaya Kingdom (modern-day southern Thailand) around 1612.

Yamada Nagamasa is alleged to have carried on the business of a privateer from 1620, attacking and plundering Dutch ships in and around Batavia (present-day Jakarta). Stories of Yamada burying his treasure on the east coast of Australia (and in particular, Magnetic Island off Townsville) persist but it is highly unlikely that Yamada would have ventured into that area as there were no trade routes in this region and the only ships to venture there were the ones blown off course during the summer storms. Furthermore, Yamada would have passed thousands of islands in the Torres Strait and Coral Sea and these would have provided safekeeping for any treasure and avoided a very long recovery voyage in the future.

The Ban Yipun colony was active in trade, particularly in the export of deer hide to Japan in exchange for Japanese silver and handicrafts (Japanese swords, Japanese lacquerware, high-quality papers). The Japanese were noted by the Dutch for challenging the trade monopoly of the Dutch East India Company (Vereenigde Oost-Indische Compagnie). The colony also had an important military role in the Ayutthaya Kingdom.

The Japanese colony was highly valued for its military expertise, and was organized under a "Department of Japanese Volunteers" (Krom Asa Yipun) by the King of Ayutthaya.

In the space of 15 years, Yamada Nagamasa rose from the low Thai nobility rank of Khun to the senior of Okya, his title becoming Okya Senaphimuk (ออกญาเสนาภิมุข). He became the head of the Japanese colony, and in this position supported the military campaigns of King Songtham, at the head of a Japanese army flying the Japanese flag. He fought successfully, and was finally nominated to Ligor (modern Nakhon Si Thammarat), in the southern peninsula in 1630, accompanied by 300 samurai.

After more than 12 years in Siam, Yamada Nagamasa went to Japan in 1624 on board one of his ships, where he sold a cargo of Siamese deer hide in Nagasaki. He stayed in Japan for three years, trying to obtain a Red Seal permit, but finally left in 1627, with the simple status of a foreign ship.