In the post, Isak thanks everyone in Newcastle for the years in the club.
"I want to express my gratitude to my teammates, the staff and all the fantastic supporters for the three unforgettable years we shared together", Isak writes on Instagram and continues:
"Together we have written history and taken the club to the place where it really belongs. It has been an honor to be part of the journey from reaching the Champions League to winning the first trophy in over 70 years"
The post received over 3,000 comments in 45 minutes.
The move became official on the last evening of the transfer window and Newcastle's own farewell to the 25-year-old was all the more brief and factual.
"Newcastle United can announce the sale of striker Alexander Isak to Liverpool for a significant British record transfer sum. The Swedish national team player joined the 'Magpies' from Real Sociedad in 2022 and made 109 appearances in all competitions", the club writes.
Isak has previously criticized Newcastle and meant that the club has broken promises.
"When promises are broken, trust disappears, the relationship cannot continue", he wrote on August 19.
Newcastle responded in a statement that the club had never promised Isak a move.