A cabinet department or prime minister's department (often called a Prime Minister’s Office or Cabinet Office) is a government body that supports the head of government and facilitates the coordination and operation of the cabinet. Unlike line ministries, it typically does not administer a specific policy portfolio, instead focusing on executive coordination, policy analysis, and administrative support. These bodies are broadly analogous in function to executive offices in presidential and semi-presidential systems, though institutional roles differ.

In many countries, such a department is called a Prime Minister's Office. In some other countries, there is a Cabinet Office. In the United Kingdom, the Prime Minister's Office is a part of the Cabinet Office; in Australia and New Zealand, there is a single department called the Department of the Prime Minister and Cabinet. In some countries, such as Germany and Poland, the department supporting the prime minister (the Chancellor in Germany) is called the chancellery.

Self-governing states and provinces within federations that are parliamentary democracies often have similar departments, such as a premier's department or, in Australia, Department of the Premier and Cabinet.