Yokoseura (横瀬浦) is a port located at the northern tip of the Nishisonogi Peninsula on the Japanese island of Kyushu, administratively under Saikai city, Nagasaki Prefecture. It was developed as an entrepot by the Portuguese in 1562 with the permission of the local lord Ōmura Sumitada, but was burned down a year later during a rebellion against Sumitada.

Ōmura Sumitada was a Japanese daimyō lord of the Sengoku period. He became famous throughout the country for being the first of the daimyo to convert to Christianity following the arrival of the Jesuit missionaries in the mid-16th century. Following his baptism, he became known as "Dom Bartolomeu". Sumitada is also known as the lord who opened the port of Nagasaki to foreign trade.