The 31-year-old is back for a second spell in west London.
Austin scored eight goals in 19 appearances for the R’s in the second half of last season, helping them to finish ninth in the table.
“I am really pleased to get it done and to be able to continue my journey here,” said the 31-year-old forward, who previously made 82 appearances for QPR before joining Southampton in January 2016.
“With the way we finished the season – and how I feel about the club hasn’t changed – I wanted to be part of it and as soon as the opportunity was available I wanted to get this done as soon as possible.
“I know this is the right move for me and the right move for my family. Where am I happiest? I was going through a period where I wasn’t enjoying my football – I came here and enjoyed it straight away.
“I love everything that the club stands for. It’s a family club and one that I’m suited to.”
QPR manager Mark Warburton added: “I am delighted we have been able to bring Charlie in.
“Everyone saw the impact he had during his loan spell, both on and off the pitch. He has played at the highest level and he knows the standards that are required if you are to be successful.”