Was Berlin Part of Prussia?

Yes. Berlin was not only in Prussia—it was the capital of Prussia for nearly 250 years.

• 1701: Frederick I crowns himself King in Prussia; Berlin becomes royal capital.
• 1701–1918: Capital of the Kingdom of Prussia (leading state of the German Empire after 1871).
• 1918–1947: Capital of the Free State of Prussia (within Weimar Republic, then Nazi Germany).
• 1947: Allied Control Council formally abolishes the State of Prussia (Law #46).

After WWII, Berlin became the divided capital of East/West Germany. Since 1990, it is the capital of reunified Germany. The Prussian legacy is visible in landmarks: Brandenburg Gate, Unter den Linden, Museum Island, and the Prussian palaces.