A grandfather shirt or grandad shirt is a long-sleeved or short-sleeved flannel or brushed cotton collarless or band-collared shirt. In Ireland, the term "grandfather shirt" is typically applied to a collarless buttoned shirt. In Britain, a collarless shirt worn by the working class was sometimes known as a "union shirt". The shirts were traditionally plain white or white with coloured vertical stripes.

"Grandfather shirts", a band-collared shirt, were historically worn by labourers. The collar-less or band-collared style was favoured by labourers as a removable collar made for easier laundering and as stiff collars were uncomfortable and unneccecssary.^{[citation needed]} Some sources suggest that the collar was removed by factory workers and labourers as collars and ties represented a safety hazard in industrial settings.

A similar style of collarless shirt, known as a "union shirt", was also worn by working-class men in Great Britain during the industrial era. At this period, the lack of a turndown or collar "cape" was filled by the use of a detachable collar or scarf, as access to laundered collars was difficult or unnecessary.^{[citation needed]}

When worn with an attached/button-on collar, to "dress the shirt up", the collared grandfather shirt was colloquially known as a "Sunday Shirt". Longer shirts were sometimes worn as a nightshirt.

In the late 20th century, grandfather shirts were associated with the skinhead subculture in Britain. In the early decades of the 21st century, the garment featured as a mainstream fashion item for men. Some sources associate the trend with the British television series Peaky Blinders.