In the Wee Small Hours is the ninth studio album by Frank Sinatra. It was released in April 1955 by Capitol and produced by Voyle Gilmore with arrangements by Nelson Riddle. The album's songs deal with themes such as introspection, melancholy, lost love, failed relationships, depression and night life; as such, it has been called one of the first concept albums. The cover artwork reflects these themes, portraying Sinatra alone at night on an eerie and deserted city street awash in blue-tinged street lights.

The album was a commercial success, cementing Sinatra's career resurgence after releasing a string of hit singles and two prior albums with Capitol, and winning an Academy Award for his role in the film From Here to Eternity. In the Wee Small Hours peaked at number two on the US Billboard 200 chart, where it stayed for 18 weeks, becoming Sinatra's highest-charting album since Songs by Sinatra in 1947. It was issued as two 10-inch LP discs, but also as one 12-inch record LP, making it one of the first of its kind in the pop field. It was also issued as four four–song 45-rpm EP discs sold in cardboard sleeves with the same cover as the LPs. The success of the album helped popularize the 12-inch LP in popular music, and the 10-inch format quickly fell into obsolescence.

Critically acclaimed since its initial release, Rolling Stone ranked it number 100 on their list of The 500 Greatest Albums of All Time in 2003, dropping it to number 101 in the 2012 revision and to number 282 in the 2020 update. Sinatra would successfully continue recording "concept" albums with later releases such as Songs for Swingin' Lovers! (1956), Where Are You? (1957), Only the Lonely (1958) and No One Cares (1960).