American Top 40 (abbreviated to AT40) is an internationally syndicated, independent song countdown radio program created by Casey Kasem, Don Bustany, Tom Rounds, and Ron Jacobs. The program is currently hosted by Ryan Seacrest and presented as an adjunct to his weekday radio program, On Air with Ryan Seacrest.

Originally a production of Watermark Inc. (later a division of ABC Radio known as ABC Watermark, now Cumulus Media Networks which merged with Westwood One), American Top 40 is now distributed by Premiere Networks (a division of iHeartMedia). Nearly 500 radio stations in the United States, the Armed Forces Network, and multiple other stations worldwide air American Top 40, and the show streams through several venues, including iHeartMedia's own iHeartRadio app.

Co-creator Kasem hosted the original American Top 40 from its inauguration on July 4, 1970, until August 6, 1988. Shadoe Stevens took over the program on August 13, 1988, and hosted until January 28, 1995, when the original program came to an end. Three years later, Kasem teamed up with Premiere's predecessor AMFM Radio Networks to relaunch American Top 40. Kasem, who had spent nine years hosting his own countdown Casey's Top 40 for Westwood One, returned to hosting his creation on March 28, 1998. Seacrest took over American Top 40 on January 10, 2004, following Kasem's retirement from the series.

Currently, American Top 40 with Seacrest airs in two formats, with one distributed to Contemporary Hit Radio (Top 40) stations and the other to Hot Adult Contemporary stations. However, there is no distinction made between the two shows on air. There are also two classic editions of the original American Top 40 distributed every weekend, featuring past Kasem-hosted shows from the 1970s and 1980s.

In its early years, American Top 40 used the Billboard charts to compile the countdown, touting it as "the only source". The program subsequently switched to being based on Radio and Records airplay data upon its late 1990s return, until R&R was folded into Billboard in 2009. The current source for the American Top 40 charts are unpublished mainstream Top 40 and hot adult contemporary charts compiled by Mediabase (who had been the chart data supplier for Radio & Records).

Here we go with the Top 40 hits of the nation this week on American Top 40, the best-selling and most-played songs from the Atlantic to the Pacific, from Canada to Mexico. This is Casey Kasem in Hollywood, and in the next three hours, we'll count down the 40 most popular hits in the United States this week, hot off the record charts of Billboard magazine for the week ending July 11, 1970. In this hour at No. 32 in the countdown, a song that's been a hit 4 different times in 19 years! And we're just one tune away from the singer with the $10,000 gold hubcaps on his car! Now, on with the countdown!

— Casey Kasem at the beginning of the inaugural AT40 broadcast

American Top 40 began on the Independence Day weekend in 1970, on seven radio stations, the first being KDEO in El Cajon, California (now KECR), which broadcast the inaugural show the evening of July 3, 1970. Billboard reported prior to the release of the show that it had been sold to stations in 10 radio markets. Kasem was reportedly inspired by Make Believe Ballroom, a radio show that was nationally syndicated in the 1940s and popularized the concept of a disc jockey playing current popular recorded music on the air.