"I'll Feel a Whole Lot Better" is a song by the Los Angeles folk rock band The Byrds that was first released in June 1965 on the B-side of the band's second single, "All I Really Want to Do". It was also included on The Byrds' debut album, Mr. Tambourine Man. Written by Gene Clark, who also sings the lead vocal, "I'll Feel a Whole Lot Better" features some of The Byrds' early musical trademarks, including Jim McGuinn's jangling 12-string Rickenbacker guitar; Clark's pounding tambourine; McGuinn, Clark and David Crosby's complex harmony singing; and a country-influenced guitar solo.

Although it was initially released as a B-side, "I'll Feel a Whole Lot Better" was itself heavily promoted by Columbia Records during the time that "All I Really Want to Do" spent on the Billboard charts. As a result, the song actually managed to chart in its own right at number 103. Since its release, the song has become a rock music standard, inspiring a number of cover versions over the years. It is also considered by many critics to be one of the band's, as well as Clark's, best and most popular songs, with Rolling Stone magazine ranking it at number 234 on their list of The 500 Greatest Songs of All Time.

Music critic Mark Deming has noted that lyrically, "I'll Feel a Whole Lot Better" takes a sardonic view of romance, with Clark undecided about whether to break off a relationship with a woman who hasn't been entirely honest with him. The song dates from The Byrds' pre-fame residency at Ciro's nightclub in Los Angeles, as Clark explained during an interview: "There was a girlfriend I had known at the time, when we were playing at Ciro's. It was a weird time in my life because everything was changing so fast and I knew we were becoming popular. This girl was a funny girl, she was kind of a strange little girl and she started bothering me a lot. And I just wrote the song, 'I'm gonna feel a whole lot better when you're gone,' and that's all it was, but I wrote the whole song within a few minutes."

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