The Federation Cup champions dominated the proceedings but they had to pay the price for their profligacy as Ruidas' 53rd minute goal handed Bengaluru FC their second successive loss.
Bengaluru FC with one point are yet to win from three matches to slip to 10th place in the 11-team table, while East Bengal (four points) climbed to fifth place.
It was Deepak Mondal, who put in a low cross to Ranti Martins but the Nigerian ace failed to direct it towards the goal as the young Ruidas deftly scored past a lunging Rino Anto to give East Bengal their first win from two matches.
Bengaluru FC could have taken a 15th minute lead but Joshua Walker's clean strike from a Chhetri pass hit the crosspiece.
Minutes later Lyngdoh had a powerful 25-yard strike as East Bengal narrowly escaped after a close save by Subhasish Roy Chowdhury.
Bengaluru FC had their best opportunity in the 40th minute when Anto set up Sean Rooney with a clinical cross but the Aussie forward fumbled twice in front of an open net as they eventually had to pay the price for the costly misses.