He is a three-time Olympian, qualifying in 2016, 2021, and most recently 2024, where he became Greece's first Olympic medalist in swimming since the inaugural 1896 Games through his 2nd place finish in the 200 metre backstroke.

At the 2013 European Junior Swimming Championships, Christou placed 3rd in the 50 metre backstroke, his first medal finish at a major competition outside his home country, Greece.

At the European Junior Swimming Championships in July 2014, Christou again competed in all three backstroke distances, placing in the top two of all three events.

In August 2015, Christou made his senior international debut at the 2015 FINA World Championship, although he failed to advance past the heats in all of his events.

Still, these earlier swims all marked significant personal bests for Christou, as he won his first senior international medal, bronze.

During the 2016 Summer Olympics in Rio de Janeiro, Christou competed in the men's 100 and 200 metre backstrokes, failing to qualify for semifinals in both, and swam on the 4x100 freestyle and medley relays for Greece, neither of which advanced to the finals.

[12] In the men's 4 x 100 metre medley relay, Greece placed 15th in the heats in a time of 3:36.75 after Christou led off with a 54.68 backstroke leg.

At the 2021 European Championships, Christou broke the 53 second barrier in the 100 metre backstroke for the first time, first swimming a 52.77 in semifinals, setting his first national record in the event,[20][18][21] before winning bronze with a 52.97.

In preliminaries, Christou swam a 48.65 100 metre freestyle (split), beating Kristian Gkolomeev's previous mark of 48.68 by .03 seconds.

[27] Christou also placed 5th in the 50 metre backstroke with a time of 24.57,[28] and these two placements would mark his highest world championships finishes to that point.

At the 2024 European Aquatics Championships in June, two months before the Olympics, Christou won gold in both the 50 and 100 metre backstroke distances in times of 24.39 and 52.23, respectively.

He was hoping to secure a medal—Greece's first in swimming since 1896—but after a disappointing 4th place finish in the 100 with a time of 52.41, just 0.02 off Ryan Murphy in bronze, Christou sought redemption in the 200.

Despite being caught by Kos after closing in 30.46, the slowest in the entire field by a significant margin, Christou was able to hang on for 2nd place, outtouching Switzerland's Roman Mityukov by 0.03 seconds.

[32] This was not only one of the most surprising medals of the entire Olympics, as Christou, known to specialize in sprint events like the 50 and 100, swam a massive personal best over 1.5 seconds faster than his seed time to get onto the podium, but also one of the most historic.

[1] As a result of this swim, Christou also reclaimed his national record, beating Apostolos Siskos's 1:55.42 from the 2024 European Championships by over half a second.