British media reported that Southampton paid four million pounds ($5.7 million) for the 26-year-old who has agreed a four-and-a-half-year contract.
“This signing is an indicator of our intent to finish the season strongly and continue the progress we’ve made in the last few years," Southampton executive director of football Les Reed said.
Southampton are 12th in the top-flight standings following an inconsistent first half of the season.
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“Charlie has a fantastic goalscoring pedigree but remains hungry to improve and develop as a footballer -- something that convinced him that Southampton is the ideal next step in his career," Reed added.
Austin scored 18 goals for QPR in the Premier League last season but they were relegated to the Championship.
“I can't wait to get started now," Austin said. "As soon as Southampton showed an interest that was the one club I wanted to go to."
(Reporting by Ed Osmond; editing by Clare Fallon)
