Obi picked up the membership card on Saturday during the flag-off ceremony of the ADC registration and mobilisation exercise in Anambra State.
The event drew party members, supporters and political stakeholders from across the state.
His move comes amid earlier speculation about a possible return to the Labour Party, the platform on which he contested the 2023 presidential election.
Obi recently declared his intention to contest the 2027 presidential election.
The former governor said he would not be intimidated by the large number of state governors currently aligned with the ruling All Progressives Congress (APC).
Obi also disclosed that several attempts had been made to discourage him from joining the race, but maintained that he would not be deterred.
His decision to formally join the ADC is expected to reshape political alignments as early manoeuvrings ahead of the 2027 presidential contest begin to gather momentum.
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