The Northampton Saints Under 18 team was targeted by thieves while they were winning e Premiership Rugby Academies League title at Allianz Park in north London.
The academy team celebrated being crowned league champions after a match at the home of Saracens Rugby club on Monday night.
But the players returned to their dressing room to find all their valuables had been stolen including a number of iPhones.
Team captain Harry Mallinder tweeted: “It has been a pleasure to captain such a good group of lads to victory. What a way to play our final game together as a group.
“Unfortunately we have had all our valuables stolen. So could you resist contacting me on my phone unless to verbally abuse the thief.
He later added: “To the pathetic cowardly thief whose face will be all over the grounds CCTV footage… You’ve got my phone… but no winner’s medal!”
Chris Wearmouth, Saints spokesman, said the theft had left a “sour taste” on what was a fantastic day for the club.
He said: “It is very disappointing that these items were stolen from the dressing room.
“It has put a sour taste on what was a fantastic day for the Saints and the players concerned. We are working closely with Saracens, who hosted the event, and Premiership Rugby, who organised it.
“We will make sure that all the items stolen will be replaced.”