His arrest and potential imprisonment before he might face trial could unleash fresh turmoil in Catalonia and threaten the fragile Socialist-led coalition ruling Spain, which relies on Puigdemont's hardline Junts party to pass legislation.
To secure Junts' legislative support for Socialist Prime Minister Pedro Sanchez's government, Spain's parliament in May approved an amnesty law designed to allow Puigdemont a safe return from Belgium.
The amnesty aimed to cancel legal proceedings against hundreds of separatists involved in Catalonia's 2017 illegal referendum, but Spain's Supreme Court ruled in July the law did not apply to an embezzlement charge against Puigdemont and upheld an arrest warrant he faces.
Still, Puigdemont said he remained committed to attending the Catalan parliament's session that is due to swear in the region's new leader following an election in May, in which Puigdemont finished second.
The swearing-in is scheduled for Thursday.
Puigdemont said he would keep his word and be in Barcelona then.
"I have started the return trip from exile," he said in a video posted on X.
He also said in the video message that his arrest would be illegal and arbitrary.
Neither Puigdemont nor Junts said on Wednesday whether he had already entered Spain.
Thursday's session is poised to appoint Socialist Salvador Illa as the new Catalan president, ending more than a decade of separatist governments.
Puigdemont's supporters plan to welcome him on Thursday morning on a Barcelona boulevard near the regional parliament, setting the stage for a potentially dramatic arrest.
Moderate separatist party Esquerra Republicana de Catalunya, currently governing Catalonia, called Puigdemont's return an "exceptional moment" and said it would attend the welcoming event.
That might require a delicate balancing act from the party, which as leader of the regional government is responsible for the regional police force that could arrest Puigdemont.
