Dom Phillips was last seen in the Javari region of Brazil at the weekend.
He was travelling the area with Bruno Pereira while working on a book. Indigenous groups say the two had received threats days before.
Mr Phillips' family say "every second counts" in the hunt for the regular contributor to the Guardian newspaper.
Federal police and navy are searching for the two in the remote region.
Mr Phillips, 57, has lived in Brazil for over a decade.
An aerial view of the Javari Valley reservation
HANDOUT
Dom Phillips and Bruno Pereira are missing
Former Brazilian President Luiz Inacio Lula da Silva
Mr Pereira is an expert on isolated tribes in the Amazon.
The two had been in the area for around a week and had travelled by boat to Jaburu Lake on Friday.
They were expected to return to Atalaia do Norte city on Sunday afternoon, according to two indigenous groups.
The pair never arrived.
Mr Phillips' wife Alessandra Sampaio said in a statement: "Please answer the urgency of the moment with urgent actions.
"In the forest every second counts, every second could be the difference between life and death."
Former Brazilian President Luiz Inacio Lula da Silva tweeted: "Phillips interviewed me for the Guardian in 2017. I hope they are fine, safe and will be found quickly."
The Guardian said staff are "very concerned" about Mr Phillips and are "urgently seeking information about Mr Phillips' whereabouts and condition".
