"We have had encouraging and positive news on Leigh's progress all along the line, but the news we have had today is good enough to ask him to join us," said Lions tour manager Gerald Davies.
Halfpenny will take the number of wings available to Ian McGeechan up to five, but his introduction will also mean the Lions gain an extra goalkicking option.
It was his ability to kick goals from the half-way line, as he did in the Heineken Cup semi-final against the Leicester Tigers, that helped convince the Lions selectors he was worth a place.
In the rarified atmosphere of the High Veld, he should be able to give the Lions some real fire power from long range, as well as a strong running option on the flank.
One player who will be delighted to see Halfpenny take the tour numbers up to 37 is Munster centre Keith Earls, who will no longer be the youngest member of the party. It means halfpenny can take over the traditional task applied to the baby of the squad of carrying the Lions mascot.