Kashiwa, promoted to the top flight at the start of the season, beat Urawa Reds 3-1 in front of 54,000 fans at Saitama Stadium on Saturday to pip defending champions Nagoya Grampus to the title by a point.
The victory, ensured by goals from Brazilian Jorge Wagner, Wataru Hashimoto and Akimi Barada, means they will join the likes of European champions Barcelona and South American champions Santos in the seven-team tournament that runs from December 8-18.
"It was a fitting finish to the season because we spent more than 80 percent of the time on top," the team's Brazilian coach Nelsinho told the Kyodo news agency.
"Today, we played the way we've been playing all year... I am very proud to be champions with this staff, with these players."
Kashiwa open their Club World Cup campaign against New Zealand's Auckland City in Toyota Stadium on Thursday with the winners going on to play Mexico's Monterrey for a chance to meet Santos in the semi-finals.
"I know we could play Santos in the Club World Cup, but we shouldn't get too far ahead of ourselves," said former Sao Paulo attacking midfielder Wagner. "We've got Auckland City first and that's who we need to think about."
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