Junior level: 2005–2010

Stefanidi attended the 1st High School of Pallini where she won the National High School Championships. She broke the national high school pole vault record and the championship record, winning gold at the 2006 Gymnasiade. Growing up she broke all of the world age-group records for the ages of 11–14 and right after she turned 15, she broke the world under-18 best with a jump of 4.37 m. Her first international experience came at the age of 15, when she represented Greece at the 2005 World Youth Championships in Marrakesh, winning the gold medal and setting a championship record of 4.30 m in the process. At the 2007 edition held in Ostrava, she was second with 4.25 m. In 2008, the 18-year-old won the bronze medal with 4.25 m at the World Junior Championships in Bydgoszcz, Poland. The same year, she gained an athletic scholarship at Stanford University and began competing for the Stanford Cardinal.^[8] She received her master's degree in cognitive psychology with Dr. Gene A. Brewer at Arizona State University while training under the guidance of 2000 Olympic pole vault champion Nick Hysong.

At Stanford, Stefanidi broke the freshman school record with a leap of 4.13 m, under the guidance of coach Kris Mack and head coach Edrick Floreal. In 2010, under the guidance of Toby Stevenson, she tied for fifth place (4.30 m) at the NCAA Division I Indoor Championships, was the Pac-10 Conference champion and tied for fourth (4.25 m) at the NCAA Division I Outdoor Championships, after breaking the school record multiple times.

2011–2013

In 2011, she was second at the NCAA Indoors with 4.40 m. That same year she defended her Pac-10 Conference title (4.28 m) and was third at the NCAA Outdoors with 4.40 m. She placed second at the European Under-23 Championships in Ostrava, Czechia and third at Universiade in Shenzhen, China, after jumping a personal best of 4.45 m in both competitions to tie the Greek U23 record.

Her senior year, Stefanidi placed third at the NCAA Indoors (4.35 m). In the 2012 outdoor season, she was again the Pac-12 Conference champion in Eugene, Oregon, where she broke her own school record with 4.48 m. A month later she became the NCAA champion in Des Moines, Iowa, clearing 4.45 m. Her season's best, 4.51 m, achieved in July in Livermore, California, is the Greek under-23 record.

In 2013, she faced some injury problems, thus not improving her personal best, with a 4.45 m season's best.

2014–2015

Stefanidi at the 2014 European Championships in Zürich

In the 2014 indoor season, she improved her personal best to 4.55 m. In the 2014 outdoor season, she improved her lifetime best to 4.57 m and then established even better best of 4.60 m at the Diamond League meeting in New York City. She competed for the first time at the European Athletics Team Championships 1st league, held in Tallinn, representing Greece and winning with a 4.55 m jump. She managed to improve her PB again at the Glasgow Diamond League in with a jump at 4.65 m. One week before the European Championships she set another personal best at 4.71 m, equaling the Greek national record.

At the 2014 European Championships held in Zürich, Switzerland she had an easy qualification, while in the final she had her first success in major events by winning the silver medal with 4.60 m, losing the gold medal in the very last jump of the event to Anzhelika Sidorova. She later won the Birmingham Diamond League meeting with 4.57 m and took the third place at the circuit's final held in Zürich with 4.67 m. With these results, she took the second place overall at the Diamond League series, only behind Fabiana Murer. During 2014, she jumped 10 times above 4.55 m, while her pre-2014 PB was 4.51 m.

During the 2015 indoor season, she set a personal best four times (4.56 m, 4.60 m, 4.61 m and 4.77 m, which was temporarily the national record). At the 2015 European Indoor Championships, she won the silver medal with 4.75 m.^[3]

2016

During training season and outside of competitions, Stefanidi trained at SPIRE Institute and Academy, a United States Olympic Training Center in Geneva, Ohio.^[9]

In the 2016 indoor season, she set a national record with a huge leap of 4.90 m at the Millrose Games, which ranked her at fourth place of all time in the event, tied with Demi Payne who cleared the same height at the same meeting. At the World Indoor Championships in Portland, Oregon, Stefanidi won the bronze medal with a jump of 4.80 m. In the following months she continuously improved her personal outdoor record (4.73, 4.75 and 4.77 m), and in Filothei, she set a Greek record of 4.86 m. In July at the Amsterdam European Championships, she won the gold medal with 4.81 m, taking the competition record from Yelena Isinbayeva.

At the 2016 Rio Olympics, Stefanidi became Olympic champion with a jump of 4.85 m, becoming the seventh Greek female athlete to win a gold medal at the Summer Olympic Games. In September, she won her first Diamond League Trophy.

2017–2018

During the 2017 indoor season, she became the European Indoor champion with a world-leading jump of 4.85 m. In the 2017 summer season, she set a world lead at the Rome Diamond League with 4.85 m and attempted for the first time to set a new world record. Two years after a disappointing performance at the 2015 Beijing World Championship, where she missed the final, Stefanidi won the gold medal at the World Championships held in London, where she even broke her own Greek record and set a new world lead for 2017 by vaulting 4.91 m. Katerina completed her 2017 outdoor season undefeated (14 wins in a row) and winning the Diamond League final in Brussels. On 14 October that year, at the traditional European Athletics gala in Vilnius, Lithuania, she was named European Women's Athlete of the Year.^[7]

In 2018, Stefanidi increased her collection of medals, taking the third place (4.80 m) at the World Indoor Championships in Birmingham, and the first place (4.85 m) at the Berlin European Championships. She won the Diamond League title for a third consecutive year and completed the season with a second place (4.85 m) behind Anzhelika Sidorova, representing Europe at the Continental Cup.

2019–present

In 2019, she won the Diamond League Trophy for a record fourth consecutive year and won the bronze medal at the 2019 World Championships in Doha, Qatar.

In 2020, as the COVID-19 pandemic shut down most public sports events, Stefanidi, along with Katie Nageotte and Alysha Newman, took part in an online event conceived by Renaud Lavillenie and organized by World Athletics called "The Ultimate Garden Clash". At their local tracks, athletes had to collect as many vault clearances in two 15-minute periods as possible with the bar set at four metres. Stefanidi won with 34 clearances.^[10] She was training at SPIRE Institute and Academy to prepare for the 2020 Tokyo Olympics.^[11] At the Games, she took the fourth place, equaling her season's best with a jump of 4.80 m.^[12]

International competitions

Representing  Greece

Year	Competition	Venue	Position	Result
2005	World Youth Championships	Marrakesh, Morocco	1st	4.30 m CR
2007	World Youth Championships	Ostrava, Czech Republic	2nd	4.25 m SB
2008	World Junior Championships	Bydgoszcz, Poland	3rd	4.25 m SB
2011	European U23 Championships	Ostrava, Czech Republic	2nd	4.45 m NU23R
	Universiade	Shenzhen, China	3rd	4.45 m
2012	European Championships	Helsinki, Finland	– (f)	NM
	Olympic Games	London, United Kingdom	24th (q)	4.25 m
2013	European Indoor Championships	Gothenburg, Sweden	13th (q)	4.36 m
2014	European Championships	Zurich, Switzerland	2nd	4.60 m
2015	European Indoor Championships	Prague, Czech Republic	2nd	4.75 m
	World Championships	Beijing, China	15th (q)	4.45 m
2016	World Indoor Championships	Portland, OR, United States	3rd	4.80 m
	European Championships	Amsterdam, Netherlands	1st	4.81 m CR
	Olympic Games	Rio de Janeiro, Brazil	1st	4.85 m
2017	European Indoor Championships	Belgrade, Serbia	1st	4.85 m WL
	European Team Championships Super League	Villeneuve-d'Ascq, France	1st	4.70
	World Championships	London, United Kingdom	1st	4.91 m WL NR
2018	World Indoor Championships	Birmingham, United Kingdom	3rd	4.80 m
	European Championships	Berlin, Germany	1st	4.85 m CR
	Continental Cup	Ostrava, Czech Republic	2nd	4.85 m CR
2019	European Indoor Championships	Glasgow, Scotland	4th	4.65 m
	European Team Championships Super League	Bydgoszcz, Poland	1st	4.70 m
	World Championships	Doha, Qatar	3rd	4.85 m SB
2021	Olympic Games	Tokyo, Japan	4th	4.80 m =SB
2022	World Championships	Eugene, OR, United States	5th	4.70 m SB
	European Championships	Munich, Germany	2nd	4.75 m SB
2023	European Indoor Championships	Istanbul, Turkey	4th	4.60 m
	European Team Championships 1st Division	Chorzow, Poland	8th	4.40 m
	World Championships	Budapest, Hungary	–	NM

Circuit wins and titles

• Diamond League pole vault Overall winner (4): 2016, 2017, 2018, 2019
  • 2014 (1): Birmingham British Grand Prix
  • 2016 (4): Rabat Meeting International (MR), Rome Golden Gala (=PB), Monaco Herculis, London Anniversary Games
  • 2017 (6): Doha Diamond League (EL), Rome (WL), London, Birmingham, Zürich Weltklasse (DLR), Brussels Memorial Van Damme
  • 2018 (2): Lausanne Athletissima (SB), Zürich
  • 2019 (3): Shanghai Diamond League (SB), Birmingham, Brussels

Progression