"I Was a Fool" is a song written and performed by Canadian duo Tegan and Sara for their seventh studio album, Heartthrob (2013).

Inspired by Rihanna's singles "Unfaithful" and "Umbrella", the song is about a woman who is in a troubled relationship but doing nothing to fix it, making her seem like a fool.

"[6] Sara wrote the bridge, with Tegan explaining in a track-by-track commentary, "There were eight or 16 bars of silence in the song—I can't remember what the number was—that I'd just left for her.

The original mix of "I Was a Fool" was produced, engineered with Jesse Shatkin, and programmed by Greg Kurstin, who also played keyboards, guitar, bass and piano, at Echo Studio.

Billy Bush recorded the drums, which were played by Joey Waronker, at EastWest Studios in Hollywood, with Andrew Ford and Jeremy Miller assisting.

The track was mixed by Manny Marroquin at Larrabee Studios with help from Chris Galland and Del Bowers.

Club and AbsolutePunk critic Drew Beringer opined "I Was a Fool" to be reminiscent of 1980s big-hair-era power ballads by Heart.

[17] Stereogum journalist Michael Nelson also honored it as the best track of the record, saying that it "sounds like an instant classic, the type of thing that should soundtrack a few million breakups over the next few years and still be prominent in our cultural consciousness three decades from now".

[18] Marc Burrows of Drowned in Sound called "I Was a Fool" one of the album's "best moments", along with "Love They Say" and "Now I'm All Messed Up", praising the lyrical content: "the heartbreak comes in waves, but it's never cloying, never over the top.

Instead it gives real punch..."[19] Sarah H. Grant, reviewing Heartthrob for Consequence of Sound, called it "magnificent", and named it an "essential track" of the album, along with "Now I’m All Messed Up" and "Closer".

[13] In a review by Pitchfork Media, Laura Snapes used the song as an example to show the LP avoided being anything generic or radio-friendly.

"[21] However, in a positive review of the album by Paste, Dan Weiss called "I Was a Fool" an "early misfire", also noting it as the only track that didn't have "one memorable phrase you won’t be able to excise from your head ‘til at least April.

that it fell "short of irony-pop and find themselves stuck in boy band territory", describing it as "sickeningly sweet and a bit too naïve".

A woman portrayed by Mae Whitman[28] is shown crying, looking at a painting signed "Sid" as well as turning her phone over after receiving a call from the same person, whose name is revealed to be Sidney.

Kyle McGovern of Spin opined on their performance of "I Was a Fool": "the Canadian pair beautifully blended acoustic guitar strums and their signature harmonies with a lilting piano line, deservedly earning cheers and whistles from the crowd.

[33] The music video was released on February 20, 2013, on Mendler's official YouTube channel and on social networking services.

Carl Smith of Sugar Space was positive about Mendler's cover and said he loved it, while also remarking that "Bridgit's given the tune an acoustic twist with a dramatic black and white video to match.

Its consists of an "icy, slow-building ambient bassline and beat" that Michael Nelson of Stereogum said to have made Tegan and Sara's vocals "somehow even more heartbreaking".