The Irishman, who was recruited during the pre-season supplemental signing period, is expected to join his new teammates at training in coming weeks.
Keane, 22, returns to Australia to resume his AFL career after playing five games in three years for the Magpies from 2019 to 2021.
He was contracted to Collingwood for 2022 but decided to remain in Ireland for the past 12 months.
Adelaide's General List manager Justin Reid said Keane, who has also played top-level hurling and Gaelic football in Ireland, would help boost the team's defensive stocks.
"We are very excited that Mark has decided to return to Australia and committed to continuing his AFL career with us," Reid said.
"We understand the travel and quarantine challenges he faced during the COVID pandemic throughout 2020 and 2021, but thankfully a lot has changed in that space since then.
"Throughout our discussions with Mark and his management he has made it clear he is eager to pursue an AFL career with the support of his family and friends and we are delighted that will be with our Club.
"Mark is super competitive, with good speed and lateral movement which we think will complement our young and developing backline nicely."
Keane, who stands 194cm and 96kg, said the timing was right for him to return to Australia and he was excited about resuming his AFL career with Adelaide.
"I never lost the passion to play the sport and when the opportunity arose to come back it was something I obviously considered very strongly," Keane said.
17°C