Francis is the third pope to visit the synagogue after John-Paul II in 1986 and Benedict XVI in 2010.
Rome's Great Synagogue is located just across the River Tiber from the Vatican, in an area still known as the Ghetto where under the orders of some of Francis's predecessors, Jews were confined for more than three centuries until their emancipation at the end of the 19th Century.
The pope arrived at 4 pm (1500 GMT) to applause and was met on the synagogue steps by Rome's chief rabbi Riccardo Di Segni.
After the deadly attacks in Paris in November, claimed by the Islamic State group, security for the pope's visit is particularly tight, with basements in the area around the synagogue searched, dustbins sealed and parking banned.
Fears of a Paris-style assault in Rome have seen visitor numbers fall, while soldiers with automatic rifles have become a common sight around the city's historic centre.
Ties between the Christianity and Judaism, which improved under both John-Paul II and Benedict XVI, have become warmer still under Francis, who has a long-standing friendship with Argentinian rabbi Abraham Skorka, with whom he jointly published a book of conversations about issues of ethics, morality and faith.
