A touching video has emerged from the US of a man serenading his grandmother on her birthday.

The footage, captured in Palm Beach County, FL, on July 27, shows the 98-year-old woman getting emotional as her grandson sings her favourite song.

''I have the most precious memories of my Grandma Jean doing everything with a song,'' the man later wrote online.

''When I was eight years old she taught me how to sing 'Unforgettable' by Nat King Cole because it was her favourite song and I was to sing it with her for her birthday every year. Hence a tradition began.''

''Grandma lost her singing voice in her late eighties and though life with all its twists and turns put us in different parts of the country I continued the tradition even if it was just by phone or Skype,'' he added.