"Apt." (abbreviation for "Apartment") is a song by New Zealand and South Korean singer Rosé and American singer-songwriter Bruno Mars. It was released through The Black Label and Atlantic Records on 18 October 2024, as the lead single from Rosé's debut studio album, Rosie (2024). "Apt." marked Rosé's first solo single in three years and her first release since departing from YG Entertainment and Interscope Records in 2023. The song was written and composed by various contributors, including Rosé and Mars. It is an uptempo pop rock and pop-punk track, featuring indie rock and electropop influences. Inspired by a South Korean drinking game, the song's chorus is built around the game's rhythmic chant of apateu (Korean: 아파트; lit. apartment).

Critics lauded "Apt." for its catchy production, broad cross-cultural appeal, and its role in promoting Korean culture worldwide. It was a commercial success worldwide, spending nine weeks atop the Billboard Global 200 and Global Excl. US charts and earning Rosé and Mars their second number-one single on both charts. In South Korea, it became both artists' first track to attain a "perfect all-kill" and peak at number one on the Circle Digital Chart, remaining at the top for seven weeks. It was the first song by a K-pop female soloist to top Australia's ARIA Singles Chart and the first Western song to top the Billboard Japan Hot 100 in over a decade. The song also topped the charts in New Zealand, Hong Kong, India, Indonesia, Luxembourg, Malaysia, the MENA region, the Philippines, Singapore, Taiwan and Turkey. It entered the top ten on the US Billboard Hot 100 and the UK Singles Chart, becoming the first song by a K-pop female soloist to do so on either.

An accompanying music video was directed by Mars and Daniel Ramos and premiered on Rosé's YouTube channel simultaneously with the single's release. The video featured Rosé and Mars as a garage band with matching black leather jackets in a pink-coloured set. It broke a number of viewership records on YouTube, becoming the fastest video by a K-pop act to reach 500 million views on the platform. Rosé promoted "Apt." with performances on The Seasons: Lee Young-ji's Rainbow, BBC Radio 1's Christmas Live Lounge 2024, and The Tonight Show Starring Jimmy Fallon. She performed the song with Mars at the 2024 MAMA Awards, where they received the Global Sensation award.