"I'm delighted to have received this call-up and I am just really looking forward to joining the squad later this week. Obviously it is disappointing for Stephen Ferris, who is a superb player, and I wish him a quick and full recovery from his injury," said Jones.
"I am anxious to help the Lions in any way I can and I recognise how this is a great honour for me and my family."
Ulster flanker Ferris scored tries against the Golden Lions and the Cheetahs in his two appearances for the tourists before picking up a knee injury. He was sent for a scan yesterday, which revealed a tear in a medial ligament.
"Ryan has been called up to replace Stephen, who unfortunately sustained a knee injury during Monday afternoon's training," explained Lions head coach Ian McGeechan.
"He twisted his right knee in a training drill and following a medical assessment it was decided that he should undergo a scan to determine the extent of the injury. He had the scan on Tuesday morning that showed he had a grade two tear to his medial collateral ligament.
"This usually requires a four to six week recovery period and is severe enough to mean he will miss the rest of the tour. Therefore with six matches left on tour after Wednesday's match against the Sharks we believe we need to fly out a replacement as cover for the backrow.
"Ryan is on the standby reserve list and of course has been playing for Wales over the last few weeks in the USA and Canada. He will fit right in as he knows a lot of the players and of course he was a Lions replacement on the 2005 tour to New Zealand.
"In fact, on that tour he showed how competitive he is as an individual by forcing his way into the Test side for all thee matches against the All Blacks."
Jones will arrive in South Africa on Thursday and will join the squad in Cape Town - nine days before the first Test in Durban.