[3][4] At the 2014 Summer Youth Olympics in Nanjing, he won a silver medal in the Mixed 4 × 100 m freestyle relay.

[7] At the 2015 Pan American Games in Toronto, Ontario, Canada, Altamir won a gold medal in the 4×200-metre freestyle relay, where he broke the Pan Am Games record with a time of 7:11.15, along with João de Lucca, Thiago Pereira, and Nicolas Oliveira.

[8][9] At the 2015 World Aquatics Championships in Kazan, he finished 15th in the Men's 4 × 200 metre freestyle relay, along with João de Lucca, Thiago Pereira, and Nicolas Oliveira.

[10][11] At the Open tournament held in Palhoça in December 2015, he obtained qualification for the 2016 Summer Olympics in the 400-metre freestyle, with a time of 3:50.32, 0.7 seconds from the South American record.

[21] At the 2019 World Aquatics Championships in Gwangju, South Korea, Brazil's young 4 × 200 metre freestyle relay team, now with João de Lucca instead of Leonardo Coelho Santos, lowered the South American record in almost 3 seconds, with a time of 7:07.12, at heats.