The Byzantine-era synagogue at Kibbutz Hamadia, just northeast of Beit Shean, preserves a well-decorated mosaic floor from roughly the 5th and 6th centuries CE. The building went through multiple phases of construction and renovation, and the floor shows artistic influences common to Jewish synagogues across Byzantine-period Palestine: geometric patterns, menorahs, and inscriptions.
The site is modest in scale and not heavily promoted, which means you may well have it to yourself. It is worth combining with the Mosaic of Rehob and the main national park for a day that covers the full sweep of Beit Shean Valley's Jewish heritage — from the Biblical period through Roman and Byzantine occupation to the Arab conquest.
Access is through Kibbutz Hamadia. Check current opening hours locally, as staffing can be irregular.