Magnussen and fellow relay team members Matt Abood and London Olympian James Roberts have been added to the Dolphins squad.
Magnussen will compete at Rio only in the relay event.
He failed to earn an individual nod after finishing third in the 50m sprint and fourth in his pet event, the 100m freestyle, in April’s Olympic trials in Adelaide.
Abood – who turns 30 this month – will become the sixth-oldest Australian swimmer to make his Olympic debut.
The Dolphins team now boasts 37 members.
The Australian men’s relay team had to wait until the end of the qualification period on May 31 to learn if they were competing at Rio.
They missed automatic Rio qualification by finishing outside the top 12 at the 2015 world titles.
The Australian relay team – completed by world No.1 Cameron McEvoy – swam a qualification time of three minutes, 12.26 seconds at the Olympic trials, ranking them third in the world over 2015-16.
We value local independent journalism. We hope you do too.
InDaily provides valuable, local independent journalism in South Australia. As a news organisation it offers an alternative to The Advertiser, a different voice and a closer look at what is happening in our city and state for free. Any contribution to help fund our work is appreciated. Please click below to become an InDaily supporter.