At the Rio de Janeiro Olympics, Chalmers won the gold medal in the 100 metre freestyle in new junior world record of 47.58, the first Australian to do so since Michael Wenden in 1968. He had also swum the fastest time in the heats, with his 47.90 s breaking his own junior world record. In the 4x100 metre freestyle relay, he contributed to Australia taking a bronze medal. In the medley relay, he had the fastest split of the field with a time of 46.72, which helped the team to win the bronze medal. Historically only Pieter van den Hoogenband (46.70 in 2003), Vladimir Morozov(46.69 in 2017), Cameron McEvoy(46.60 in 2015) and Duncan Scott (46.14 in 2019 and Jason Lezak (46.06 in 2019) have been faster in textile swimwear.