A major factor of contact amongst the people of the Western Niger Delta of Nigeria, which for want of a more appropriate term, maybe described as cultural, between the Isoko and her Ukwuani neighbours is the religio-ritual factor. One of the ancient and wide-ranging religio-ritual factors, which helped to promote intergroup contact between the Isoko and her Ukwuani neighbours, was associated with the Mmonwu masquerade, a socio-cultural institution. The work therefore examines the impact of the Mmonwu masquerade on the relations between the Isoko and Ukwuani people of the western Niger Delta of Nigeria. The work was carried out with the use of primary and secondary sources, using the historical and analytical method. The result/findings shows that, the Isoko-Ukwuani Mmonwu Masquerade is not an original Isoko-Ukwuani culture, but came as a cultural intercourse with their Igbo Neighbours. The work further shows that the Mmonwu Masquerade has a viable socio-cultural, religio-ritual, and economic impact on intergroup relations of the Isoko-Ukwuani Peoples.