Lions tour manager Gerald Davies commented: "We are delighted that Leigh has rejoined the group, having seen the medical team this morning they have confirmed that he has arrived fully fit.
"So he is available for selection following tomorrow night's game against the Golden Lions."
Halfpenny added: "It's just great to be back with the squad. 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!"
Munster and Ireland starlet Earls, who made his Lions debut in last Saturday's win over the Royal XV, is 21 years of age and nearly 15 months older than new arrival Halfpenny.
Earls made some early errors in the tour's opening game and his woes were compounded when he picked up a shoulder injury during the first half of the 37-25 Lions victory.
He damaged his right shoulder at the clavicle region when Royals prop Albertus Buckle landed heavily on him.
Earls is back training and should be in contention to feature in the squad for Saturday's game against the Cheetahs.
Reflecting on the Limerick youngster's performance, Lions backs coach Rob Howley said: "Keith had one of those games that we've all had - we've all got the T-shirt. He was disappointed.
"He is young, he made mistakes, but I'm sure he will bounce back.
"Every player on this Lions tour has gone out and found that, for whatever reason, the ball hasn't bounced their way.
"It happened to Keith on Saturday and we are not hiding away from that fact.
"Wearing the Lions badge can sometimes make you feel 6ft 8in tall, but then you go out and drop the first ball and all of a sudden you are trying to get your game back on track.
"You could see Keith's frustration when he kicked that ball out.
"He will be a 10 times better player for that experience and I'm sure he will bounce back and show why he was picked for the tour in the first place. The player himself and his team-mates will pull him through."
Earls' centre partner on the day, Wales' Jamie Roberts, is confident he will recover to play a big role during the tour.
"The only way you learn is by playing. I was pleased to be playing alongside Keith to form such a young centre pairing.
"I'm sure he wouldn't have been pleased with some of the dropped balls, but that's the way it goes some days.
"I'm sure he will get the chance to put the jersey on again and do it justice, because he is a hell of a player."