Walker, who sang with British artist Robbie Williams, picked up more votes to beat rival Elin Bergstrand at a packed Ericsson Globe in Stockholm.
"It's unreal! Unreal! Unbelievable!" he told Reuters immediately after his victory.
While games had to be rescheduled due to Walker's participation in the show and some critics said he was receiving special treatment, fans continued to vote for him week after week to keep him in the competition.
The Idol victory will be some consolation for Walker after a disappointing end to the season with Sundsvall, who lost a playoff and missed out on promotion to Sweden's top flight.
Asked whether he would now be retiring from football to concentrate on his singing career, Walker said he had no plans for the future as yet.
"We'll see, we'll see. Tonight I'm just going to enjoy the party."
(Reporting by Philip O'Connor; Editing by Peter Rutherford)
