"This Old Heart of Mine (Is Weak for You)" is a song by The Isley Brothers from 1966 that was a hit for them during their brief tenure on Motown's Tamla label. Featuring Ronald Isley on lead vocal, "This Old Heart of Mine" peaked at number twelve on the Billboard Hot 100, and at number six on the Billboard R&B Singles chart.

In the UK, the song originally reached number 47 in April 1966, but it re-charted in late 1968 and reached number three for two weeks in November, making it the group's highest charting UK single.

Written by Motown's main songwriting team Holland–Dozier–Holland alongside Sylvia Moy, "This Old Heart of Mine", produced by Brian Holland and Lamont Dozier, was originally intended for The Supremes (who later recorded their own version for their Supremes A' Go-Go album). The single was the group's only major hit while on Motown, whom they left in 1969 to restart their own T-Neck label. Tammi Terrell also recorded and released the song as a part of her Irresistible album released in January, 1969.

The song was prominently featured in an episode of the hit 1980s show Moonlighting and was one of the tunes included on the show's hit soundtrack.

In 1975, Rod Stewart released a remake of this song that charted in several markets (U.S. #83). In 1989, Stewart released another version--this time a duet of the song with Ronald Isley that reached number 10 on the US Billboard Hot 100 chart and spent five weeks at #1 on the US adult contemporary chart. The later version was produced by Bernard Edwards and Trevor Horn and was released on Warner Records.

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