"Knock You Down" is the fourth U.S. single from Keri Hilson's debut album, In A Perfect World.... The song features rapper Kanye West and R&B singer Ne-Yo.

The music video for the song premiered on March 23, 2009, one day before the release of the album. The song will be released as the second single in the United Kingdom on 29 June 2009.

The song is Hilson's highest charting single as a lead vocalist in the United States, peaking at #13 on the U.S. Billboard Hot 100 chart.

It was written by Nathaniel Hills, Marcella Araica, Kevin "KC" Cossom, Shaffer Smith, Keri Hilson and Kanye West. Like most of the songs on the album Keri Hilson made final amendments to the song before it was recorded. Danja executively produced the single. Hilson said that she had always wanted to work with Ne-Yo and Kanye West, and West's vocals were a last minute addition to the single. It was confirmed on Hilson's official video blog album countdown.

The song has proven to be Hilson's most successful single worldwide, being her first single to enter the AIRA 50 chart and surpassing the peak of her song "Return the Favor" on the UK charts as well her highest charting single in Canada and the U.S.

The music video for "Knock You Down" was shot in Los Angeles, California with director Chris Robinson.

In the video's first scene Hilson is filmed free-falling in the bedroom of her apartment. The next scene follows her life with Kanye West playing her love interest, after which Ne-Yo takes over playing another love interest during his verses. The final part of the video features a split screen which on the left shows what is happening in the present when West sees Hilson making out with Ne-Yo while the right side of the screen relays the first part of the video in black and white. The final scene is a small confrontation between her two love interests and the video ends with Hilson's free-falling scene shown again. There are no specific dance routines featured in the video.

It was premiered on Access Granted on March 23, 2009. Nick Tershay of Diamond Supply Co. makes a cameo in the video during Kanye West's verse. San Diego Chargers linebacker Shawne Merriman also makes a cameo.

On May 14, the video made its UK premiere on Channel Four and 4 Music, quickly soaring up video request charts and radio airplay