Former Manchester United and Atletico Madrid striker Forlan had failed to find the net in his first three top-flight appearances in Japan, but struck four minutes from time at the Kashima Soccer Stadium to seal a big win for his new club.
Aria Hasegawa opened the scoring for the visitors after 18 minutes, before Uruguay international Forlan struck to ensure Cerezo's third win in a row and end Kashima's 100 per cent winning start to the campaign.
That result leaves Kashima and Cerezo on nine points apiece, and allowed the latter to leapfrog Yokohama F Marinos into second spot on goal difference as last season's runners-up slipped to a shock 1-0 reverse at Ventforet Kofu.
A rare goal from veteran defender Katsuya Ishihara's midway through the second half was enough for Ventforet to claim a maiden victory of the season.
Nagoya Grampus kept pace with the top three with their third successive league victory, which they had to earn the hard way by coming from behind to beat newly-promoted Vissel Kobe 2-1.
Pedro Junior broke the deadlock for Vissel after just nine minutes, only for Ryota Isomura to respond almost immediately for the hosts.
Riki Matsuda then stole all three points for Akira Nishino's side with a goal two minutes from time.
Meanwhile, title holders Sanfrecce Hiroshima's inauspicious start to the defence of their crown continued, as they needed a second-half penalty from Hisato Sato to take a point from their trip to last season's J2 League champions Gamba Osaka.
At the other end of the table, Tokushima Vortis's difficult start to life in the J.League continued as they were beaten 2-0 at home by Kashiwa Reysol. The second-tier play-off winners' record in the top flight now reads played four, lost four, conceded 12, scored none.
Vegalta Sendai and Tokyo are also awaiting their first wins of the season after they were both defeated 4-0 on Sunday by Omiya Ardija and Kawasaki Frontale respectively.
Elsewhere, Urawa Reds missed the opportunity to join Kasima, Cerezo, Yokohama and Nagoya on nine points as they were held 1-1 at home by Shimizu S-Pulse, while Leo Silva's goal on the half-hour mark was enough for Albirex Niigata to overcome Sagan Tosu 1-0.
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