Phillip Cocu's side have been top of the table since the second game of the season and, despite pressure from Ajax in recent weeks, secured the championship with three games to go.
De Jong opened the scoring after three minutes with a well-placed header and the Philips Stadion erupted when Memphis Depay doubled their lead with a stunning free-kick.
Heerenveen pulled a goal back through Joey van den Berg, but De Jong's second and a late Luciano Narsingh strike ensured all three points – and the title – for Cocu's men, who lead Ajax by 13 points.
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