IU is one of the highest-earning celebrities in South Korea, with endorsements attributing to a proportion of her income. Over the course of her career, she has endorsed various products that ranged from electronics to clothing and cosmetics. Her breakthrough in 2010 earned her the first three endorsement deals she had with Crown Confectionery's MyChew candy, Crown Bakery and clothing brand Y'sb. Among her first endorsements that year were also video games. When developer Ntreev Soft had chosen IU to endorse MMORPG Alicia: The Story of My Horse and I, they cited her "simple and innocent" image had complemented with the game's vibe, and featured her in the soundtrack. IU announced the player introductions for the Global StarCraft II League — she later performed at the finals of the league in 2011.

IU has continuously been featured by Samsung for its advertisements for products such as Anycall, Galaxy S II, Wave 3, and Samsung Card in 2018–19. Samsung also added a feature to have her voice for the Celeb Alarm in late-December 2019. Additionally, IU was the ambassador for Expo 2012 held in Yeosu. Following the release of A Flower Bookmark, she participated in Sony Korea's promotional campaign for its MDR-series headphones with You Hee-yeol and continued to be a representative for the brand's audio products through 2015. IU has modeled for clothing brands Unionbay and Le Coq Sportif from 2011, the SK Planet online store 11st, G by GUESS with Yoo Seung-ho, Elite with Infinite, SBENU with Song Jae-rim, and J.Estina jewelry. Gucci appointed IU as its brand ambassador for South Korea in February 2020, and later as global brand ambassador in March 2022, she has posed with its clothing in numerous pictorials. Later in July 2021, New Balance also announced IU as its global brand ambassador and started partnering for its "We Got Now" campaign. As for cosmetic brands, IU has endorsed the SAEM, Qdsuh in China, ISOI, and CNP Cosmetics. After IU returned to endorse Unionbay in 2015 with Lee Hyun-woo and ISOI selected her as a model for two years, the two entities sponsored The Producers, in which IU had a leading role, through product placement.

In April 2022, IU was selected as an advertising model by Woori Financial Group, owner of one of the four largest banks in South Korea, for her influence across various generations. That same month, the MAU (monthly active users) using its services increased by 100,000, reaching 5.8 million users. The next month, it exceeded 6 million users, an increase of about 500,000. A spokesperson of Woori said that they believed it was an effect of IU's endorsement. Other endorsements include S-Oil, Homeplus, Nongshim's products, fast-food chain Mexicana Chicken, and Jeju Samdasoo and Binggrae Banana Flavored Milk both in mid-2020. In November 2014, HiteJinro announced that IU had become the spokesperson for Chamisul soju. They renewed a contract with her in December 2015. In April 2024, Estée Lauder announced IU as its first Korean global brand ambassador.

She was the female model with the most advertising appearances on television in 2024, starring in commercials for 13 different brands.