Asser (; Welsh: [ˈasɛr]; died c. 909) was a Welsh monk from St David's, Dyfed, who became Bishop of Sherborne in the 890s. About 885 he was asked by Alfred the Great to leave St David's and join the circle of learned men whom Alfred was recruiting for his court. After spending a year at Caerwent because of illness, Asser accepted.

In 893, Asser wrote a biography of Alfred, called the Life of King Alfred. The manuscript survived to modern times in only one copy, which was part of the Cotton library. That copy was destroyed in a fire in 1731, but transcriptions that had been made earlier, together with material from Asser's work which was included by other early writers, have enabled the work to be reconstructed. The biography is the main source of information about Alfred's life and provides far more information about Alfred than is known about any other early English ruler. Asser assisted Alfred in his translation of Gregory the Great's Pastoral Care, and possibly with other works.

Asser is sometimes cited as a source for the legend about Alfred's having founded the University of Oxford, which is now known to be false. A short passage making this claim was interpolated by William Camden into his 1603 edition of Asser's Life. Doubts have also been raised periodically about whether the entire Life is a forgery, written by a slightly later writer, but it is now almost universally accepted as genuine.