Pope Francis, 88, died on Monday after suffering a stroke and cardiac arrest. In February he was admitted to a Rome hospital for double pneumonia and was discharged late last month.
Luxon is in the United Kingdom, having wrapped up meetings with King Charles III and Prime Minister Sir Keir Starmer.
He is about to head to Gallipoli in Turkey for Anzac Day services on Friday, marking the 115th anniversary of the World War 1 landings.
The Prime Minister will then go straight to St Peter's Basilica in the Vatican to represent New Zealand at the Pope's funeral, which begins on Saturday night (NZ time).
Luxon said he would join other world leaders mourning the loss of Francis, while celebrating the pontiff's service to the more than 1 billion Catholics across the world, including almost 450,000 in New Zealand.
Prince William will attend the funeral on behalf of his father, King Charles, Kensington Palace said.
New Zealand Cardinal John Dew will take part in the conclave process at the Vatican which will determine the Pope's successor.
