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

"I Was a Fool" was released in Canada as the second single from the album on April 24, 2013, and peaked at nineteen on Canadian Hot 100 and number one on CBC Radio 2 Top 20, making Tegan and Sara the first twins in history to take the top spot on the chart. In July 2013 "I Was a Fool" was certified Gold in Canada.

In November 2011, Tegan and Sara announced that they were writing new material for their seventh studio album, with Tegan saying that "[we] are back in our home studios working away at more songs. We’re really looking forward to making a new record now. Our hope is to be in the studio by early 2012." Tegan composed "I Was a Fool" and wrote the verses and chorus. She began the composition process on a guitar, then moving it to the piano. She was inspired to write a piano ballad after listening to Rihanna's 2006 single "Unfaithful": "I was listening to [it] and wanted to create a similar piano riff." She also wanted "I Was a Fool" to be "something really sad and kind of fucked, but something that everybody would relate to", in the vein of Rihanna's "Umbrella".

As Tegan recalls writing the lyrics, "I was in Los Angeles where I have a place with my girlfriend and I was messing around on the guitar. We ran out to do some errands and halfway there, I said to my girlfriend, 'We have to go back.' I had the line 'I did behave' stuck in my head so I went home and recorded that one part. It sat on my computer for about a week then I sat down to continue writing and the rest of the song came together really fast." 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. She called me and was like, "That is impossible! You need to put a guitar or something in there." So I ended up just using the same chords from the chorus, and she sang overtop of it."

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