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Disclaimer: Swim of the Week is not meant to be a conclusive selection of the best overall swim of the week, but rather one Featured Swim to be explored in deeper detail. The Swim of the Week is an opportunity to take a closer look at the context of one of the many fast swims this week, perhaps a swim that slipped through the cracks as others grabbed the headlines, or a race we didn’t get to examine as closely in the flood of weekly meets.
Japan’s Yumeki Kojima has been rewriting the record books in 2025, most notably breaking the World Junior Record in the boys’ 400 IM last month in a time of 4:09.38.
The most incredible part of that performance was that Kojima is just 16 years of age—with his 17th birthday coming on December 20—and he’ll be eligible to set World Junior Records all the way up until December 31, 2026.
A few weeks before he broke the WJR in the 400 IM, Kojima clocked 1:57.77 in the 200 IM at the Aichi Prefectural Swimming Championships in early July, which marked the fastest performance ever from a 16-year-old.
At this week’s World Junior Championships in Otopeni, Romania, Kojima continued his run of success, producing a time of 1:57.32 in the final of the boys’ 200 IM to re-lower the fastest mark ever recorded by a 16-year-old.
Sitting just shy of his personal best pace through 150 meters, Kojima closed strong, more than half a second quicker than he did in early July, to knock more than four-tenths off his all-time mark for 16-year-olds.
Split Comparison
| Kojima, July 2025 | Kojima, August 2025 |
| 25.08 | 25.13 |
| 54.45 (29.37) | 54.28 (29.15) |
| 1:28.94 (34.49) | 1:29.05 (34.77) |
| 1:57.77 (28.83) | 1:57.32 (28.27) |
Kojima led the field at the final turn, and found himself in a neck-and-neck battle with Russian (representing Neutral Athletes B) swimmer Mikhail Shcherbakov on the last 50, with Shcherbakov edging him out for gold in a time of 1:57.25, breaking the Championship Record set by American Maximus Williamson in 2023 (1:57.29).
In addition to Shcherbakov and Kojima’s impressive swims, the heat also featured Kojima’s Japanese teammate, Raito Numata, who is also 16 and clocked 1:57.98 to become the 2nd-fastest 16-year-old ever.
Kojima owns three of the four fastest performances ever from a 16-year-old, having also clocked 1:58.47 back in March 2025. He and Numata are the only 16-year-olds to own any of the top 1000 performances of all-time.
Shcherbakov’s 1:57.25 and Williamson’s 1:57.29 are also the only swims from a 17-year-old that are quicker than Kojima’s time at 16, though it’s worth noting that the performances from all three came in their 17th year, with Kojima the only one who hadn’t had their birthday at the time of the performance.
Prior to the start of World Juniors, Williamson ranked #2 all-time among 16-year-olds with his 1:58.65 from 2023.
For additional context, former Japanese star Kosuke Hagino, a four-time Olympic medalist, clocked 1:57.35 at the age of 17.
In the all-time junior rankings, including all 18 & under boys by birth year, Kojima now ranks #6 all-time.
The fastest-ever junior time belongs to Michael Phelps, who clocked 1:55.94 at the age of 18, while Hungarian Hubert Kos holds the official World Junior Record of 1:56.99. Phelps’ best time at 16 was 2:00.86.
All-Time Performers, Junior Boys’ 200 IM (LCM)
- Michael Phelps (USA), 1:55.94 – 2003
- Hubert Kos (HUN), 1:56.99 – 2021
- Qin Haiyang (CHN), 1:57.06 – 2017
- Mikhail Shcherbakov (RUS), 1:57.25 – 2025
- Maximus Williamson (USA), 1:57.29 – 2023
- Yumeki Kojima (JPN), 1:57.32 – 2025
- Carson Foster (USA), 1:57.59 – 2019
- Matt Sates (RSA), 1:57.60 – 2021
- Raito Numata (JPN), 1:57.98 – 2025
- Ilia Borodin (RUS), 1:58.00 – 2020
Adding to the superlatives of the 200 IM final on Wednesday, China’s Xie Yichen, who only turned 15 on August 1, clocked 1:59.36 to place 4th.
In addition to his silver in the 200 IM, Kojima swam the lead-off leg on Japan’s gold medal-winning mixed 4×100 medley relay, recording a 100 back time of 54.90, and he’ll be the big favorite to win individual gold in the 400 IM on Saturday. He’s also slated to contest the 200 back, entered with no time, but after his 54.9 lead-off leg on the medley relay, he’ll surely be dangerous.
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