Secret Service Director Kimberly Cheatle is throwing in the towel on her job at the federal agency -- this after the bungled attempted assassination of Donald Trump ... and all the scrutiny that's followed, TMZ has confirmed.

Cheatle tendered her resignation Tuesday effective immediately, just a day after she was on Capitol Hill getting grilled by lawmakers over the bungled security detail with Trump at his rally in Pennsylvania ... where he was shot by Thomas Matthew Crooks.

Multiple members of Congress demanded answers from Cheatle on what exactly went wrong that weekend -- and most of her responses were vague or unsatisfactory to the House members ... with many of them demanding she exit stage left since the buck stops with her.

One of the key issues from Monday's hearing pertained to how long it would take to get to the bottom of what happened and why Crooks was able to get on the roof and take a shot at the former president.

The Secret Service has given itself 60 days to investigate ... but nobody on Capitol Hill was happy with that timeline -- nor were they pleased with Cheatle not having adequate answers on specifics to Trump's detail that day ... not to mention the communication breakdown.

Cheatle had previously said she takes responsibility for the shooting -- but in the same breath, she also said she felt the security presence around DT that day was adequate.

Clearly, since then, he's been assigned more agents ... and amid massive outcry, she's now out.