Cristiano Ronaldo had held the record after appearing in the Euro 2004 final against Greece at the age of 19 years and 150 days.
But at 18 years and 326 days, Sanches overtakes his Portugal team-mate as Fernando Santos’ side look to beat France.
Sanches also overtook Ronaldo earlier in Euro 2016 by becoming the youngest player to play for Portugal at a major tournament.
And by finding the net against Poland in the quarter-final, the midfielder also became the youngest player to score a goal in the knockout stages of the European Championship.