Blanche of Lancaster (March 25, 1345 - September 12, 1369 Bolingbroke Castle) was an English noblewoman, daughter of Henry of Grosmont, 1st Duke of Lancaster by his wife Isabel de Beaumont. Both she and her elder sister Maud, Countess of Leicester were born at Bolingbroke Castle in Lindsey.

On May 19, 1359, Blanche was married to John of Gaunt, a son of the reigning English king Edward III of England and his Queen consort Philippa of Hainault. The title Duke of Lancaster became extinct upon her father's death without male heirs in 1361. However, through his marriage to Blanche, John of Gaunt became Earl of Lancaster, Earl of Derby, Earl of Lincoln and Earl of Leicester (although Gaunt would not receive all of these titles until the death of Blanche's older sister, Maud, in 1362). The Duchy of Lancaster (second creation) would later be bestowed on Gaunt. The influence associated with the titles would lead him to become Lord High Steward of England.

Bubonic plague struck the Kingdom of England for the third time in 1369, and among the dead was Blanche Lancaster. Her husband was at sea at the time of her death. He held annual commemorations of her death for some years thereafter. For one of these, Geoffrey Chaucer, then a young squire and mostly unknown writer of court poetry, was commissioned to write what became The Book of the Duchess, in her honour. Though Chaucer’s intentions can never be defined with absolute certainty, many believe that at least one of the aims of The Book of the Duchess was an attempt to make John of Gaunt see that his grief for his late wife had become excessive and to prod him subtly to move on.

In 1374, five years after her death, John of Gaunt ordered effigies made of himself and his wife. Twenty-five years later, Gaunt was laid to rest next to Blanche, the two are buried somewhere in St. Paul's Cathedral in London.