"I'll Feel a Whole Lot Better" is a song by the Los Angeles folk rock band the Byrds, first released in June 1965 on the B-side of the band's second single, "All I Really Want to Do". Despite initially being released as a B-side, the song managed to chart in its own right in the U.S., just outside the Billboard Hot 100. It was also included on the Byrds' debut album, Mr. Tambourine Man.

The song was written by band member Gene Clark, who also sings the lead vocal."I'll Feel a Whole Lot Better" has been covered by a number of different artists over the years, and is regarded by fans and critics as one of the Byrds' best known songs.

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."