"Knock Knock" is a song recorded by South Korea-based girl group Twice for their special album Twicecoaster: Lane 2 (2017). The song was released on February 20, 2017 as the album's lead single and title track by JYP Entertainment. It was written by Shim Eun-ji, Lee Woo-min "collapsedone", and Mayu Wakisaka.

On January 18, 2017, it was reported that Twice would be releasing a special album after their concert in Seoul. In early February, Twice officially announced the release of the reissue of Twicecoaster: Lane 1, titled Twicecoaster: Lane 2, on February 20 with a newly recorded track titled "Knock Knock".

Two music video teasers for "Knock Knock" were released on February 17 and 18 at midnight. The first teaser started with the sound of knock which was heard on the ending of the music video for "TT", while the second teaser showed the members dressed in a variety of outfits that flash by in the stop-motion video. The final group image teaser was released on February 19 at midnight. Twice also greeted the fans via Naver V Live for a comeback countdown at 23:30 KST. The music video for "Knock Knock" was officially released the next day in conjunction with the release of the album. It was also released for digital download on various music sites.

The music video for "Knock Knock" was directed by Naive Creative Production. Stop-motion effect was used in some of the scenes in the video. It earned almost 10 million views on YouTube within 24 hours since its release and set a new record for the fastest K-pop group music video to reach 30 million view count in only 152 hours. It was also the most viewed K-pop music video for the first half of 2017.

A sense of innocence was portrayed in the music video, which featured Twice having fun in a slumber party and snowball fight. Producer Park Jin-young, founder of JYP Entertainment, also had a short appearance. The last scene was the same ending as the music video of "TT".

"Knock Knock" was a commercial success in South Korea. It debuted atop the Gaon Digital Chart for the week ending February 25, 2017, accumulating 319,957 downloads and 7,136,193 streams within its first week of release. The following week, the song dropped to number two on the Gaon Digital Chart, selling 141,204 downloads and gaining 6,965,178 streams. The single additionally peaked at numbers 5 and 15 on the World Digital Songs and Japan Hot 100, respectively.

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