Up and coming athlete Joseph Burnell, who turns 15 today, had come fifth overall in his event at Roundhay Park on Saturday.
He is due to compete again later this month but this has been thrown into doubt after his endurance road bike was stolen in the early hours of yesterday.
His mum, Andrea Burnell, said: "Joseph is part of the Yorkshire talent squad at the Brownlee Centre. He was selected and has been training there since October last year.
"It was his first triathlon on Saturday at Roundhay Park. We put the bike away, had a barbecue with family and friends, and at 2am yesterday morning our garage was broken into."
Tadcaster Grammar School pupil Joseph had come second in his wave and fifth overall in his category, but the family's joy turned to heartbreak when the theft was discovered.
Footage captured on a neighbour's CCTV cameras shows the burglars trying vehicle doors as they make their way along the street shortly after 11pm.
When they return into shot at 2am, one of the pair is seen cycling down the middle of the road on Joseph's bike.
Andrea, of Birchfields Crescent, Leeds, said: "They've used the tools that we had stored in our garage to cut through the bike lock."
She said forensics teams had visited to gather evidence and the CCTV footage was being passed on to police.
The bike is a matt grey and black Cannondale Synapse Claris with white writing on the frame.
"All I want is for the bike to be returned," Andrea said. "That's all we're interested in.
"Joseph has got a triathlon in two week's time and he's got no bike now."