Yuliya Andreyevna Yefimova (Russian: Юлия Андреевна Ефимова, also romanized Efimova; born 3 April 1992) is a Russian competitive swimmer. She is the Russian record holder in the 200 metre individual medley (short course), 50 metre breaststroke (short course and long course), 100 metre breaststroke (short course and long course), and 200 metre breaststroke (short course and long course). After making her Olympic debut in 2008, she went on to win the bronze medal in the 200 metre breaststroke in 2012, and silver medals in the 100 metre and 200 metre breaststroke in 2016. She is a six-time World Champion, winning the 50 metre breaststroke in 2009 and 2013, the 100 metre breaststroke in 2015, and the 200 metre breaststroke in 2013, 2017, and 2019. In 2019, she became the first woman to win the 200 metre breaststroke at a FINA World Aquatics Championships three times. She is a former world record holder in the long course 50 metre breaststroke. She has won 109 medals, including 48 gold medals, at Swimming World Cups.

After failing a drug test, Yefimova was disqualified from competition for 16 months, from October 2013 to February 2015, was stripped of her results and medals at the 2013 European Short Course Championships, and four of her world short-course records were invalidated.

Yefimova was born in Grozny. However, due to the First Chechen War, her family moved to Volgodonsk soon after. There she took up swimming at the age of six, coached by her father Andrey Yefimov. Until 2011, she lived in Taganrog, where she trained under Irina Vyatchanina and studied at the Southern Federal University. In March 2011, she moved to California, United States, where she was coached by Dave Salo, the head coach of the University of Southern California swimming team.

Yefimova's first notable achievement was winning the 50 metre, 100 metre, and 200 metre breaststroke titles at the 2007 European Short Course Swimming Championships. At 15 years of age, she set her first European record and Russian national record in the short course 200 metre breaststroke with her time of 2:19.08 in the final that won her the gold medal in the event. At the 2008 European Aquatics Championships she was the gold medalist in the 200 metre breaststroke and the silver medalist in the 50 metre breaststroke. Yefimova competed at the 2008 Summer Olympics in Beijing when she was 16, placing fourth in the 100 metre breaststroke and fifth in the 200 metre breaststroke. In the prelims heats of the 100 metre breaststroke, she ranked second overall, swimming a new European record and Russian record time of 1:06.08 and finishing behind only Leisel Jones of Australia. At the 2009 World Aquatics Championships in Rome, Italy, Yefimova won the gold medal in the 50 metre breaststroke in a world record time of 30.09 seconds at 17 years of age.

Yefimova won gold medals at the 2010 European Aquatics Championships in the 50 metre and 100 metre breaststroke events. The following year, at the 2011 World Aquatics Championships, she won the silver medal in the 200 metre breaststroke with a Russian record time of 2:22.22, finishing less than one second behind the gold medalist in the event Rebecca Soni of the United States. In 2012, she earned the bronze medal in the 200 metre breaststroke at the 2012 Summer Olympics with a new European record time of 2:20.92. A year later, at the 2013 World Aquatics Championships in Barcelona, Yefimova triumphed in the 200 metre breaststroke with a time of 2:19.41. In the 50 metre breaststroke prelims heats, she achieved a new world record of 29.78 seconds. She won the finals, finishing first in 29.52 seconds.

In January 2014, it was announced that Yefimova had failed an out of competition drug test in October 2013. Her positive test was for DHEA, an endogenous steroid hormone banned in professional sports. On 13 May 2014, she was disqualified by FINA for 16 months, from 31 October 2013 to 28 February 2015. She was stripped of her results and medals at the 2013 European Short Course Championships. Her four world short-course records (two relays and the 50 metre and 200 metre breaststroke set in November/December 2013) were also invalidated.

In August 2015, after 16 months of disqualification due to the positive drug test, Yefimova competed at the World Aquatics Championships in Kazan. She won a gold medal in the 100 metre breaststroke with a time of 1:05.66 that was seven tenths of a second faster than the next fastest swimmer, Rūta Meilutytė of Lithuania. In the 50 metre breaststroke Yefimova won the bronze medal at 30.13 seconds. In the 200 metre breaststroke she placed 17th in the prelims heats with a time of 2:26.11 and did not advance to the semifinals. She also placed fifth in the 4 × 100 metre mixed medley relay, splitting a 1:05.46 for the breaststroke leg of the relay.

Between 15 February and 30 March 2016, Yefimova tested positive for meldonium six times in total. She was provisionally suspended from international competition by the International Swimming Federation (FINA) on 14 March. With no research on how long the drug stayed in a person's system, she was not banned or given a suspension. In June 2016, Yefimova returned to competition at the Los Angeles Invitational, winning the 200 metre breaststroke with a time of 2:22.77.