There is speculation he may announce a unilateral declaration of independence following a disputed referendum.
Catalan police have been posted outside parliament in Barcelona, sealing off the grounds to the public.
The mayor of Barcelona has urged Mr Puigdemont and Spanish PM Mariano Rajoy to “de-escalate” the crisis.
Mr Puigdemont’s address, which is due at 18:00 (16:00 GMT), comes after a vote was held on 1 October which Catalan officials say resulted in almost 90% of voters backing independence. Turnout was put at 43%.
The vote was deemed illegal by Madrid and suspended by Spain’s Constitutional Court. “No” voters largely boycotted the ballot and there were several reports of irregularities. National police were involved in violent scenes as they manhandled voters.
