Napoli's 3-0 victory over already-relegated Pisa sealed a top-four Serie A finish with one game to spare, making the transfer automatic under the terms agreed when Højlund joined on loan last August for an initial fee of six million euros.
Manchester United will therefore receive a total of 50 million euros from the deal, confirmed by transfer journalist Fabrizio Romano.
Højlund played a central role in Sunday's decisive win, setting up Scott McTominay for the opening goal before scoring in stoppage time to seal the result. It was his 15th goal of the season in all competitions across 43 appearances, a significant recovery in confidence and output after a difficult two years at Old Trafford.
He wrote on Instagram after the match: "This goal felt symbolic because it means two things. We now secured Champions League with Napoli, which means because of my contract I'm now officially a Napoli player and I'm saying goodbye to Manchester United. That feels weird, because for the past year I already feel like a Napoli player. The way you fans supported me, made me feel like home and let me find my confidence again is something I'm so grateful for."
He also addressed his time at United directly.
"To say goodbye to Manchester United is something that kinda feels emotional too. My childhood dream came true by playing at Old Trafford in a red jersey. So the goal today symbolises a new confident start, in which I will give everything for the time ahead to Napoli, and a thank you to all the fans, players and staff of Manchester United who made the dream I had as a kid come true. It's time for new dreams, so let's chase them."
Napoli sporting director Giovanni Manna had stated earlier in the season that the club intended to keep Højlund regardless of their Champions League status. United paid 64 million euros to sign him from Atalanta in August 2023. He scored 26 goals in 95 appearances in all competitions across two seasons at Old Trafford. Both United and Napoli will compete in next season's Champions League, raising the prospect of a reunion in the group stage.