The 20-year-old Cardiff wing had been one of six Blues named in Ian McGeechan's original party but, after Tom Shanklin was forced to withdraw with a dislocated shoulder, Halfpenny himself suffered an injury setback prior to departure.
Halfpenny was required to undergo rehabilitation on a quad injury back in the Welsh capital with the Blues medical team and did not step on to the plane to South Africa with the rest of the squad 10 days ago.
After arriving in Johannesburg three days after the first match against the Royal XV, Halfpenny underwent a post-flight recovery session with the Lions medical team and will now play a full part in the tour.
Halfpenny will be available for selection against the Cheetahs on Saturday and will still have plenty of time to play his way into the Test team for the three-match series against the Springboks beginning on June 20.
"We are delighted that Leigh has rejoined the group," said tour manager Gerald Davies.
"Having seen the medical team this morning, they have confirmed that Leigh has arrived fully fit and so is available for selection following tomorrow night's game against the Golden Lions."
"It's just great to be back with the team," added Halfpenny.
"I've been working really hard with the Cardiff medical team and they have got me back to full fitness so I am raring to go for the Lions.
"Also, as the youngest player on tour I now have responsibility for Lenny the Lion, so Keith (Earls) officially handed over custody as soon as I walked into the team room!"
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