Replying to Pakistan's 250, spearheaded by opener Nasir Jamshed's second consecutive hundred of the series, India were bundled out for 165 in 48 overs as they failed to break their Eden Gardens jinx against Pakistan.
As Junaid Khan shattered Ishant Sharma's stumps with a snorter of a delivery, the visitors got their fourth win in as many ODIs against India at the iconic ground. India skipper Mahendra Singh Dhoni remained unbeaten on 54.
With the hosts losing the opener by six wickets in Chennai, the final match in Delhi is now reduced to a mere formality. This is Pakistan's first ODI series triumph in India since the 4-2 win in 2005.
It was a fine show from pacers Junaid (3/39) and Umar Gul (2/24), who backed up Jamshed's efforts with their variations and movement rattling the Indian willowers.
Much to the dismay of a capacity crowd, the Indian top order showed its brittleness for the second straight match, with half the side back into the pavilion even before reaching 100.
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India lost the plot there and literally gave up the chase. It was only a painful display to extend the proceedings at the end with Pakistani bowlers building the pressure up to a crescendo.
Openers Gautam Gambhir (11) and Virender Sehwag (31) looked tentative against the beauty of deliveries sent down by Junaid and Mohammed Irfan but managed to put on a stand of 42.
However, Gambhir, who never looked confident, inside edged Junaid to the stumps, thereby starting the procession.
A short while later Virat Kohli (6) nicked Junaid behind the wicket while Gul picked up Sehwag by getting him plumb with a moving delivery.
Gul continued the momentum by evicting Yuvraj Singh (9) as India tottered at 70/4 in 18.5 overs.
The strokes also dried up. After a boundary from Suresh Raina's blade in the 24th over, the Indian batsmen could not send the ball across the ropes for the next 101 deliveries.
Spinner Saeed Ajmal took three late wickets -- seeing the back of Ravindra Jadeja (13), Bhuvaneshwar Kumar (0) and Ashok Dinda (2) -- before Dhoni and Sharma added 33 for the last wicket.
Earlier, Man of the Match Jamshed fired a 124-ball 106 (12x4, 2x6) to give Pakistan a rollicking start before India fought back to run through the visitors' batting line-up.
Riding on the hundred by the 23-year-old Jamshed -- who completed a hat-trick of hundreds against India -- and his 141-run opening wicket stand with Mohammed Hafeez (76), Pakistan looked poised for a 300-plus total after being put into bat. But they surrendered wickets at regular intervals to fold up in 48.3 overs.
Except the opening stand, there was not a single half-century partnership in the innings.
Jamshed seemed to have started from where he left off in Chennai reaching his century off 120 balls in the 40th over, driving Ravichandran Ashwin for a boundary past mid-off.
The only blemish in Jamshed's innings came on 35 when he edged one to backward point but Kohli failed to grip the offering. Hafeez initially looked circumspect but grew in confidence as the overs progressed.
Ultimately, left arm spinner Jadeja got the breakthrough. Hafeez missed a sweep and lost his wicket.
Skipper Misbah-ul Haq (2) did not last long while Jamshed's knock ended in the 41st over. The left-hander darted down the track to Jadeja, missed, and Dhoni rattled his stumps in his second effort, reducing Pakistan to 210/5.
Azhar Ali (2) was run out, while Younis Khan (10) got an unfortunate leg before decision. Television replays showed the ball striking the pad off an inside edge. Pakistan were 177/3 then in 34.3 overs.
The young Jadeja celebrated by sending Kamran Akmal (0) back two balls later.
Ishant Sharma (3/33), luckless till then, struck gold in his last spell, removing Shoaib Malik (24), Umar Gul (17) and Mohammad Irfan (0) within nine balls.