Sammie Szmodics and Liam Delap scored in the first half to hand Ipswich a two-goal advantage at the Tottenham Hotspur Stadium; Rodrigo Bentancur had one back in the 69th minute, but the North London outfit were unable to find a second.
The win was Ipswich's first in the league this season, which moved them out of the bottom three, but it was a fifth league defeat of the campaign for Spurs, who are now 10th in the Premier League table.
Postecoglou has taken the blame for the home reverse, claiming that he is "not getting consistent performances from the players" at this moment in time, with Spurs winning only two of their last five matches in all competitions.
"Hugely disappointing. We started slow and passively. We gave ourselves a mountain to climb. Second half we had chances but didn't do enough to win the game," the Spurs head coach told BBC Sport after the clash.
Tottenham suffered fifth league defeat of the season vs. Ipswich
"It's just down to me. I'm not getting consistent performances from the players. It's something I need to address. I'm the person in charge so that's usually the way it goes. I take responsibility.
"We can't give ourselves those kind of mountains to climb."
Meanwhile, in a separate interview with Sky Sports News, Postecoglou said that he would have to wait until after the November international break to fully address the situation.
"They all go away so there's nothing I can address in the next 13 days until I get them back," said the manager.
Son opens up on "painful" defeat
Spurs captain Son Heung-min, meanwhile, branded the defeat "very painful".
"It's very disappointing. Before they scored we had a couple of chances. We conceded a very sloppy goal that shouldn't happen. It's more than disappointing," he told reporters.
"Sometimes you have to be calm and find a better solution if someone is in a better position. We have to be better in those kind of situations. It's very painful."
Tottenham were also beaten 3-2 by Galatasaray in the Europa League, making it successive losses for the capital giants ahead of the November international break.
Spurs will return to action against wounded English champions Manchester City at the Etihad Stadium on November 23, before ending the month with a Europa League clash against Italian outfit Roma.