"If You Don't Know Me by Now" is a song written by Kenny Gamble and Leon Huff and recorded by the Philadelphia soul musical group Harold Melvin & the Blue Notes, which became their first hit after being released as a single in 1972, topping the R&B chart and peaking at number three on the Pop chart.

The song was originally written for Labelle (a trio led by Patti LaBelle) but they never recorded it. After some revisions, the song was offered to The Dells but they also never recorded it. Harold Melvin & the Blue Notes signed to Gamble & Huff's own Philadelphia International label in 1971 and they did record the song, with the late Teddy Pendergrass as lead vocalist. In addition to the single release, the song was included on their eponymous debut album.

Patti LaBelle made the song part of her concert repertoire in 1982. A live version appears on her 1985 album, Patti.

It was later covered by the English pop/soul band Simply Red, also becoming their best-known hit after reaching number one on the U.S. Hot 100 on July 15, 1989 and at number thirty-eight on the Hot Black Singles chart. It peaked at number two in the UK Singles Chart. It also topped the Canadian Singles Chart. Seal recorded the song for his 2008 album Soul, and, in April 2009, it became his first top-ten Adult Contemporary hit since "Love's Divine" in 2004; the song was subsequently nominated for the Best Male Pop Vocal Performance Grammy

The song was chosen as one of the Songs of the Century by the RIAA. It was featured at the end of Michael Apted's movie Class Action.

It was comically covered by British comedian Ricky Gervais in his character David Brent. A music video was produced and released with BBC's The OfficeDVD set. American Idolcontestant LaToya London additionally covered the song in the season compilation album American Idol Season 3: Greatest Soul Classics.

R&B/Pop singer Ciara sampled the chorus of the song for her 2009 single, "Never, Ever", featuring Young Jeezy. The single is from her third album Fantasy Ride.

This version of the song appears on the game Karaoke Revolution Presents:American Idol.