Buda Castle Tunnel, Road tunnel beneath Castle Hill in Budapest, Hungary.

The Buda Castle Tunnel runs 350 metres beneath Castle Hill, linking the Clark Ádám tér end of the Széchenyi Chain Bridge with Krisztinaváros. Its eastern portal, in eclectic classicist style, has a round-arched opening framed by stepped voussoirs, a carved coat of arms above and rows of tall rectangular windows; a dark metal railing crowns the flat top. The roadway is 6.9 metres wide, and a sidewalk runs along the southern side.

Dániel Novák proposed a tunnel under the hill in 1837; István Széchenyi pushed the idea in 1842 and founded a joint-stock company on 1 December 1845. Adam Clark drew up the plans from 1852, and on 10 February 1853 boring began from both sides, meeting on 25 October after seven and a half months. The tunnel opened to pedestrians in 1856 and to carriages in March 1857; it was repaired by 1949 after damage in the 1944 battle for Budapest.

The eastern portal, in eclectic classicist style, carries a carved coat of arms above the arch, with a small sculpted head at the arch's apex. Until 1918 users paid a toll: one kreuzer for pedestrians, three for a one-horse carriage and six for a two-horse carriage. From 1857 until 1873 cab passengers were handed umbrellas against water dripping from the leaky ceiling.

A sidewalk runs along the tunnel's south side, but a posted sign advises pedestrians against using it because of car noise and polluted air. The tunnel is closed to all traffic for washing twice a year, in spring and autumn, usually from 11 pm to 4 am over three or four nights. Bus routes 16, 105, 178 and 216 pass through it.

The service apartment of the Chain Bridge's bridgemaster stands at the tunnel's Clark Ádám tér end, not inside the eastern portal. The same person was responsible for both the bridge and the tunnel, linking the two structures operationally.