Charteris was contacted on holiday in America and is expected to fly back to London and from there directly to South Africa.
The 2.06m tall lock has seven caps and last played for Wales as a 55th minute replacement in the inaugural Prince William Cup clash against South Africa at the Millennium Stadium last November.
Bradley Davies has left the squad's Cape Town training base to fly home to the UK still suffering from ankle ligament problems.
He was hurt in a training session on Saturday and initially it was hoped he would recover enough to be able to train with the squad next week before the second Test match against the Springboks.
Wales Assistant Coach Robin McBryde said: "We all feel very sorry for Bradley but there is never a right time to get injured. He has already proved to us that he is one for the future and I know he has benefited enormously from his time here with the squad.
"Bradley has had a chance to test himself in the environment we are creating and he is aware of the intensity and focus we are looking for.
"Luke has been involved with Wales in the past so we are confident he will slot in very quickly with the squad. This is a great chance for him to impress us and we are looking forward to seeing him arrive in South Africa."
Wales Team Manager Alan Phillips said: "We tracked Luke down to San Francisco and he is absolutely delighted to be joining us. He will fly from the USA back into London so fortunately he will have a chance to collect his boots before heading out here.
"It is never nice to see anyone leave the squad through injury but Bradley and Luke will both benefit from the experience of being involved."
Luke CHARTERIS (NEWPORT GWENT DRAGONS - Cap No 1,030)
DoB: 09.03.83. 6'9" / 2.06m. 17st 3lbs / 109kgs
International record: 7 caps (W 4, L 3)