Step 1: Understanding the Question:
The question asks to identify the most recent country to be admitted as a member of the United Nations.
Step 2: Detailed Explanation:
The membership of the UN has expanded over the decades. The most recent admission occurred in 2011.
The Republic of South Sudan was admitted to the United Nations on 14 July 2011, following its declaration of independence from Sudan.
It became the 193rd member state of the UN.
Step 3: Final Answer
The newest member country of the United Nations Organisation is South Sudan.