"Fireflies" is the first single from electronic artist Owl City's Ocean Eyes. Relient K vocalist Matt Thiessen is featured as a guest vocalist in the song. He described it as "a little song about bugs and not being able to fall asleep at night." The single has attained worldwide success, topping the charts in the United States, Australia, Denmark, Ireland, Sweden, Norway, the United Kingdom, India and the Netherlands.

The music video for "Fireflies" was directed by Steve Hoover. It features Adam Young playing the song on his old (80's) synthesizer in a toy-filled bedroom, where most of the toys (including an astronaut, toy cars, and a blimp) come to life. Most of the toys are older model toys, with most of them from the 1970s and '80s (an exception being a Robosapien). There are also vintage household devices such as an analog television and record player. The video had an exclusive premiere on MySpace, but was leaked onto Dailymotion some time before and YouTube soon after.

The song debuted on the Hot 100 in early-September at number ninety-seven, and it reached number one in its tenth week, becoming Owl City's first number-one single. The song stayed at number one for two non-consecutive weeks, in the top ten for fifteen weeks and on the Hot 100 for twenty-seven weeks so far. "Fireflies" contributed to sales of the album Ocean Eyes, and was credited as being responsible for its entry to the top ten on the U.S. 200. On the Billboard Hot 100 2009 year-end chart, it was ranked sixtieth. The RIAA certified "Fireflies" double-platinum in January 2010.

The song entered the UK Singles Chart at number fifty due to early download sales from a fake version that got leaked onto iTunes. The song would go on to make a forty-eight place jump to number two the following week, beaten only to the top by "Replay" by Iyaz. The following week, it rose to number one, and topped the chart for three consecutive weeks. The BPI certified "Fireflies" silver in February 2010

The song has attained success worldwide. It has reached number one in Australia, Denmark, Ireland, Sweden and the Netherlands(for 10 weeks) and the top ten in Austria, Belgium, Canada, Czech Republic, Finland, Germany, Japan, New Zealand, Norway, Switzerland and the Slovak Republic.