The 28-year-old, who has cystic fibrosis, has won four caps for his country and recently had a spell with French Top 14 side Clermont.
He has spent most of his career with Western Force in his homeland, but had a six-week stint at Gloucester in 2011.
"I'm ready to hit the ground running in what is a crucial few weeks in Europe and the Premiership," Charles said.
Bath director of rugby Todd Blackadder added: "After losing three hookers in the last month, we're really fortunate to be able to bring in someone of Nathan's calibre."
Bath have signed Australia international hooker Nathan Charles on a deal until the end of the season.