West Australian man Jesse Bruzzese, 35, went missing in Mexico on June 3.
A social media appeal says Mr Bruzzese's last known location was in Riviera Maya, a resort town just south of Cancun along the Caribbean coastline.
"Jesse has been travelling overseas and we've been unable to confirm where he is or whether he is okay for 3 weeks," Robert Bruzzese wrote on Facebook on Monday.
He asked that anyone who has been in touch with Mr Bruzzese or has information to contact himself or police.
A separate social media post, from a woman claiming to be a family friend, said the family was becoming increasingly concerned for his welfare.
"We are growing more worried as each day goes by," the woman, who claimed to be friends with Mr Bruzesse's brother, wrote.
"Jesse, if you happen to see this, please know that we love you so much and we are worried about you."
AAP has contacted the owners of both accounts.
Mr Bruzzese's disappearance has sparked a global search effort involving the Department of Foreign Affairs and Trade, Western Australia Police and local Mexican authorities.
The Fiscalía General del Estado de Quintana Roo, the state Attorney-General's office, shared a missing person's alert on its website.
Interpol has issued a yellow notice, an international alert used to track down missing or abducted people.
It describes Mr Bruzzese as a dual Australian and Italian national, with brown eyes and hair, 176cm tall and speaking only English.
He has several tattoos on his left arm, shoulder and chest and is likely to have visited Cambodia, Mexico, the US, Thailand, Singapore, Hong Kong, China and Indonesia.
WA Police is assisting DFAT with its inquiries, a spokesperson told AAP.
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