The 24-year-old initially joined The Tykes on a month's loan back in late November.
Since arriving at Oakwell, Monk has made 12 appearances for Gudjon Thordarson's side, scoring one goal.
He will continue with his loan spell in South Yorkshire, having been on the periphery of Saints' first team in the last few years.
Meanwhile, Norwich City boss Nigel Worthington has insisted that striker Iwan Roberts will not be joining Barnsley on loan.
The veteran marksman has been linked with a temporary switch to Oakwell, with South Yorkshire neighbours Sheffield Wednesday also said to be interested - but Worthington says he is going nowhere.
"We're going to try and drive forward with what we've got - there's nobody going on loan anywhere," Worthington told the club website.
"I've said it many times before, but to have someone of his experience about the place - particularly to come in when Svensson gets injured - is very, very handy."
