The 32-year-old agreed terms to the end of the season at St Mary's after yesterday's 4-1 victory over Aston Villa and will be added to the south coast side's Premier League squad list.
"I'm looking forward to playing in the best league in the world while I'm playing here at Southampton," Boruc told the club's official website, saintsfc.co.uk.
"I missed football, so I'm really glad that I'm here and I've finally found the club I was looking for.
"My dreams are coming true. I've been talking about this for a long time, and finally I'm here in the Premier League."