The synagogue was originally known as the New or Great Synagogue. Only at the end of the 16th century, with the founding of further synagogues in the ghetto, did the name Old-New (German: Altneuschul) come into use. One Prague legend offers a different explanation: the foundation stones of the synagogue were said to have been brought by angels from the destroyed Temple in Jerusalem, on the condition — in Hebrew, al-tenaj — that they would be returned when the Temple was rebuilt.
The exterior of the synagogue is plain, with buttresses and triangular gables decorated with relief carvings of grapevines. The building has a saddle roof with a distinctive crenellation and twelve dormers, symbolising the twelve tribes of Israel. Above the entrance is an early Gothic tympanum adorned with a vine motif.
The interior consists of a two-aisled hall whose vaulting is supported by two octagonal pillars. The unique five-ribbed vaulting is entirely exceptional in medieval synagogue architecture; the fifth rib was added deliberately to avoid any resemblance to the Christian cross pattern. At the centre of the hall stands the bimah — a raised platform with a lectern for the reading of the Torah — surrounded by an ornate Gothic grille dating from the 15th century. The Torah is kept in the aron ha-kodesh (Torah ark) by the eastern wall. Along the northern wall hangs the red banner of the Prague Jewish community, bearing the Star of David and the traditional motif of a hat, donated to the community in 1716 by Emperor Charles VI. Along the southern wall stands the raised seat of Rabbi Loew, traditionally identified as the place where the celebrated scholar used to sit.
The building is associated with a number of legends. According to one of the most famous, the remains of the Golem — a clay creature created and brought to life by Rabbi Loew to protect the Prague Jewish community — are stored in the synagogue's attic. Another legend holds that during fires the synagogue was protected by angels transformed into doves, and indeed it has survived the centuries without serious damage. Over the years the synagogue has undergone several restorations, the most significant of which was carried out by the architect Jiří Meitner in 1883. During the Nazi occupation services were suspended and the synagogue's spaces were used as a warehouse. After the war, religious life was restored and the synagogue returned to its original purpose.
The Old-New Synagogue stands in Červená Street in Josefov and is accessible as part of the Prague Jewish Quarter circuit. The synagogue is closed on Saturdays and on Jewish holidays.