Unter den Linden ("under the linden trees") is a boulevard in the centre of Berlin, the capital of Germany. It is named for its linden (lime in British English) trees that line the grassed pedestrian mall between two carriageways. Unter den Linden runs east–west from the Brandenburg Gate to the former site of the imperial palace (Berliner Stadtschloss) where the Palast der Republik is currently under deconstruction (2007). The boulevard crosses the River Spree and continues as Karl-Liebknecht-Straße. Major north–south streets crossing Unter den Linden are Friedrichstraße and Wilhelmstraße.

Unter den Linden is at the heart of the historic section of Berlin dating from the 17th and 18th centuries. It connects Pariser Platz and Bebelplatz, crossing the Schlossbrücke (palace bridge), the Lustgarten and Museum Island. Buildings along the street include (from west to east) the Hotel Adlon, the Russian Embassy (former Soviet Embassy), the Berlin State Library, the Berlin State Opera, Humboldt University, Cathedral of St. Hedwig at Bebelplatz, the Kronprinzenpalais (former palace of the crown princes), the Neue Wache war memorial, the Zeughaus Berlin (old armory; now houses the German Historical Museum (DHM)), the Alte Kommandantur and the Berliner Dom.

Well-known statues of Frederick the Great, Alexander von Humboldt, Wilhelm von Humboldt, and several Prussian generals also adorn the street.

A boulevard of linden trees was planted from 1647 extending from the electoral palace to the gates of the city by Friedrich Wilhelm, the “Great Elector” who wanted to beautify the route from his castle to his hunting park, the Tiergarten. By the 19th century, as Berlin grew and expanded to the west, Unter den Linden became the best-known and grandest street in Berlin. During the last days of World War II most of the linden trees were destroyed or cut down for firewood. The trees were replanted in the 1950s .