In the final day of league action, Reysol, who won Division Two a year ago to gain promotion to the top flight under Brazilian boss Nelsinho, became the first team to win the two divisions back to back in the 19-year history of the J.League.
At Urawa's Saitama Stadium, Kashiwa needed three points to be sure of the title, with their closest rivals Nagoya Grampus and Gamba Osaka chasing them, one point and two points behind respectively.
Brazilian attacker Jorge Wagner and Wataru Hashimoto scored to give Kashiwa a 2-0 lead at half-time.
Midfielder Yosuke Kashiwagi cut Urawa's deficit to 2-1 with his strike in the 53rd minute and the hosts went on to put the visitors under pressure.
In the 76th minute, however, Akimi Barada collected a Urawa defender's clearance and fired a shot from outside the box which proves too good for Reds goalkeeper Nobuhiro Kato.
"From the start of the season, we've been working hard to reach this point," said Nelsinho.
"I am so delighted to finish the season at the place where we should be. This is a result of the daily efforts of our players."
Kashiwa edged out defending champions Nagoya Grampus by one point and former Asian champions Gamba by two points, with both teams winning their final game on Saturday.
Nagoya beat 10-man Albirex Niigata 1-0 thanks to former Japan international Keiji Tamada's goal from a free-kick, preventing a seventh straight loss in Niigata for Nagoya.
Nagoya's Australian forward Josh Kennedy did not score on the day but finished as the league's top goalscorer for the second year in a row, with 19.
Nagoya manager Dragan Stojkovic said: "The (title) race was very interesting until the end of the season. Kashiwa Reysol deserved to be the champions. They were in the first place. We have won the last six games but were not good enough. But we played good football and we have no regrets. We are proud of ourselves."
Gamba rallied past hosts Shimizu S-Pulse 3-1 as South Korea international Lee Keun-ho scored two goals with Takahiro Futagawa adding one.
The top three finishers - Kashiwa, Nagoya and Gamba - will play in next year's AFC Champions League along with the Emperor's Cup winners, which will be determined on January 1.
If either Kashiwa or Nagoya do the double by winning the season-closing event, the league's fourth-placed side - Vegalta Sendai - will take part in the Asian club tournament. Sendai beat Vissel Kobe 2-0 at home with Shingo Akamine's double.
In other matches, Yokohama F. Marinos were held to a 1-1 draw by visiting Kashima Antlers to drop down from fourth to fifth, behind Sendai on goal difference. Kashima finished sixth.
Sanfrecce Hiroshima won 3-1 at Montedio Yamagata; Jubilo Iwata beat visiting Kawasaki Frontale 2-1; Cerezo Osaka romped past Avispa Fukuoka 7-1 at home; and Omiya Ardija downed Ventforet Kofu 3-1 at home.
Next season FC Tokyo, Sagan Tosu and Consadole Sapporo will play in the J.League after winning promotion, with Ventforet, Avispa and Montedio relegated.