The Prince of Wales was pictured pumping his fists in the air before celebrating with Aston Villa's players in the dressing room after they reached the Europa League final.
Villa fan William was photographed cheering from the stands during their 4-0 victory against Nottingham Forest, which secured a 4-1 aggregate win and set up a meeting with Freiburg in the final on May 20.
Aston Villa manager Unai Emery confirmed William had joined in with post-match celebrations.
"He was in the dressing room with the players and with me, and of course, he is so happy as well," the Spaniard said.
William was joined by club ambassador Ahmed Elmohamady in the Villa Park stands in Birmingham.
Chris Wood's penalty in the first leg had given Vitor Pereira's Forest side the edge but Ollie Watkins levelled the aggregate scores in the 36th minute.
Forest failed to deliver the type of performance that saw them go 10 games unbeaten and they went behind in the tie 13 minutes after the interval, courtesy of Emi Buendia's penalty, before John McGinn's double sealed a 4-1 aggregate win and a trip to Istanbul.
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William, patron of the Football Association, is a passionate Villa supporter and has followed the Midlands club since he was a schoolboy.
He was born less than a month after Villa's most famous continental success, when they beat Bayern Munich to lift the European Cup in May 1982.
It is not yet known whether the prince will travel to Istanbul for the final later this month.
