Kim qualified for the women's 400 m freestyle, as South Korea's youngest swimmer (aged 15), at the 2012 Summer Olympics in London, by eclipsing a FINA B-standard entry time of 4:15.01 from the Dong-A Swimming Tournament in Ulsan. She challenged seven other swimmers on the second heat, including three-time Olympian Kristel Köbrich of Chile. Kim finished the race in last place by nearly 25 seconds behind Singapore's Lynette Lim, posting the second-slowest time of 4:43.46. Kim failed to advance into the final, as she placed thirty-fourth overall in the preliminary heats.