Yes, PartyCasino is a legit online casino owned and managed by Entain PLC. It operates in New Jersey under the New Jersey Division of Gaming Enforcement license. PartyCasino is licensed in New Jersey with Borgata as its official land-based partner.
Elsewhere, players have created a current list of 54 countries that can't access Diamond Casino's gambling features. It's wide-ranging, including China, North Korea, Iceland, Cyprus and Poland.
