Alexander Isak said it had "been an honour" to play for Newcastle as he thanked the club's supporters in a farewell message following his move to Liverpool.
The Sweden striker joined the Merseyside club on deadline day on Monday for a British record transfer fee of £125m after a drawn out transfer saga.
Isak scored in Newcastle's 2-1 win over Liverpool in the Carabao Cup final in March as the club won their first domestic trophy in 70 years.
However, in the summer he said he wanted to leave the Magpies and has not played for them this season.