Joseph Saul Nathansohn (1808–1875) (Hebrew: יוסף שאול בן אריה הלוי) was a Polish rabbi and posek, and a leading rabbinical authority of his day.
He studied Talmud at Lviv (Lemberg) together with his brother-in-law Mordecai Zeeb Ettinger.
In the 1830s in Lemberg—then under the rule of the Austrian Empire—he founded an informal study-group under his tutelage; this yeshiva attracted some of the most brilliant students in Galicia.
He was a widely recognized rabbinical authority, and was asked to rule on various contemporary issues; his rulings are still widely cited (for instance he was one of the first to permit the use of machinery in baking Matzah, which created a widespread halachic controversy[1][1]).
More can be seen in Hebrew in the front of Sefer Shoel Umeshiv reprinted by Mechon Harrai Kedem and haggada shel pesach Divrei Shaul reprinted by Mechon Meoros Hatorah.