Sixers president Rod Thorn said last week he'd match any reasonable offer made to Philadelphia's top reserve. But they never let negotiations get that far, signing him on Friday, the first day of training camp, without having to match another team's offer.
Young posted on Twitter he had signed a deal.
"Promise to continue to play hard with pride and all my heart. I love Philly."
The team later announced it.
Young averaged 12.5 points over his first four seasons with the Sixers.
The Sixers took Vucevic, the 7-foot, 260-pound center out of USC, with the 16th overall selection. The Sixers stayed local in the second round, picking Temple forward Allen with the 50th pick.
The Sixers training camp roster stands at 15.
Information from The Associated Press contributed to this report.