"Thunder" is a song by British singer and songwriter Leona Lewis from her fifth studio album, I Am (2015). The song was written by Lewis and Toby Gad, who also produced the track. Lyrically, "Thunder" is about finding the strength to pick yourself up after falling on difficult times. Lewis explained that the song is about self-reflection and self-empowerment and finding the ability to look within in order to decide what is best to do next. Def Jam released "Thunder" on 24 July 2015 as the lead single in United States, but it is the third single to be released from the album overall. Remixes by Danny Verde and Tom Swoon were also released.

The track garnered a positive response from music critics, many of whom complimented its powerful and anthemic stance and declared it an album highlight. One critic believed the lyrics to be directed at Simon Cowell, Lewis' former label boss. "Thunder" reached the top thirty on the U.S. Billboard Adult Contemporary and Adult Pop Songs charts, and the top twenty on the Dance Club Songs chart. The song's accompanying music video shows Lewis in a variety of outdoor spaces as clips of varying weather patterns are intercut. She has performed the track on several U.S. television shows, including Good Morning America, The Late Late Show with James Corden and The Talk.

"Thunder" was written by Leona Lewis and Toby Gad, with the latter producing the track, for Lewis' fifth studio album, I Am (2015). It was published by ASCAP and Atlas Music publishings (on behalf of itself and Gadfly Songs (ASCAP).) Recorded and mixed by Gad at Kite Music Studios in Los Angeles, Lewis provided all of the vocals for "Thunder" and Gad performed all of the instrumentation and programming. The track's audio was mastered by Tom Coyne at Sterling Sound studios in New York City, with assistance from Randy Merrill. For the acoustic version of "Thunder", Lewis' additional vocals were recorded by Adam Lunn at Wendy House Studios in London. On 30 June 2015, Lewis released a three-minute teaser video with snippets of the title track, "Ladders, and "Power". Media site Idolator exclusively premiered the full length version "Thunder" on 16 July. Lewis told Idolator "'Thunder' is a song that is so close to my heart. I wrote it about the strength of rising after the fall and I’m thankful to be able share it." In an interview for Next Magazine, Lewis further elucidated upon the songs meaning: "It was like coming from a place where you’ve been through a hard situation that has really made you reflect or look deep inside to figure out what it is that you want to do next," said Lewis. "I had that breakthrough where now I feel strong in what I’m doing and strong in my decisions. I’m back and I’m ready to go for it."

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