Secret

Secret is a song written by The Pierces, which served as the theme song for Pretty Little Liars and played during the opening sequence. The song was originally suggested by one of the show's stars, Ashley Benson, who played Hanna Marin.^[1]

Additionally, a cover of Secret by Denmark + Winter serves as the theme song for Pretty Little Liars: The Perfectionists and plays during the opening sequences.

Pretty Little Liars Lyrics

Got a secret
Can you keep it?
Swear this one you'll save
Better lock it, in your pocket
Taking this one to the grave
If I show you then I know you won't tell what I said
Cause two can keep a secret if one of them is dead.

Pretty Little Liars Connections

A lot of fans identified "Secret" as the most appropriate theme song for The Pretty Little Liars television series before the show even aired. They found it fitting that one of the secret-keepers in the song is named Alison and that one of the two secret-keepers is dead and taking a secret with her to the grave. Additionally, the show itself revolves around secrets and lies which is the premise of the song.
Ashley Benson, a fan of The Pierces, heard the song and pitched it to the producers and creator of Pretty Little Liars. They eventually went ahead and made it the show's theme song.
The lyric "two can keep a secret if one of them is dead" was said by Alison except she added in "only" between can and keep in the Season 2 episode, Over My Dead Body. It means that if people are keeping a secret among themselves, one person can guarantee that the secret gets kept by killing the others, something A has done or attempted to do on multiple occasions toward the liars and many other characters. Very many of the killings happen in the night though no one knows why. Some people say it's because of the souls that mourn during the evening.

Pretty Little Liars: The Perfectionists Lyrics

If I show you
Then I know you
Can't tell what I said
Cause two can keep a secret
If one of them is dead.

Pretty Little Liars: The Perfectionists Connections

During the pre-production phase of the series, co-creator I. Marlene King wanted to honor the sister series by incorporating the original theme song. However, the fellow producers didn't want to use the same song, which led them to ask "Denmark + Winter" to record an eery remix of the song.
The lyric, "if one of them is dead" was the title of the Season 1 episode, "...If One of Them is Dead".

Notes

The Pierces describe their music, on ‘Thirteen Tales of Love & Revenge,’ as sounding like “a fever induced, gypsy dream while listening to a cabaret singer in a southern speakeasy that is run by witches.”
The song was also featured in Gossip Girl, in the episode Hi, Society.
The music video ^[2]
Two episodes in the Pretty Little Liars Universe are named after a line in the song: Taking This One to the Grave (PLL) and ...If One of Them is Dead (PLL: TP)