Yitzchak Blazer (1837 – 1907), also known as Reb Itzelle Peterburger, was one of the early important leaders of the Musar movement, a Jewish ethical movement based in Lithuania within the Russian Empire. He was a student of the founder of the movement, Yisrael Salanter, and was responsible for publishing many of Salanter's letters in Or Yisrael ("The Light of Israel"). Blazer became the chief rabbi of St. Petersburg in 1861-2, when he was only 25 years old. From 1880 to approximately 1891, he served as the head of the Kovno Kollel in Kaunas, Lithuania, which was founded by Salanter. Under Blazer's direction, the kollel came to be "considered by its contemporaries as a bastion of the Mussar movement," and was attacked by the Musar movement's opponents. He later joined Nosson Tzvi Finkel in leading the Slabodka Yeshiva, and eventually moved to Palestine where he died in 1907.