A work by 17th century master Anthony Van Dyck is among three "very high value" paintings stolen from a University of Oxford art gallery.
A Soldier On Horseback by the Flemish artist, a leading court painter in England under King Charles I, dates from around 1616.
Two other works were stolen from Christ Church Picture Gallery, St Aldates, at about 23:00 GMT on Saturday.
Police said a "thorough investigation" was under way to recover the paintings.
The burglars also made off with A Boy Drinking (c. 1580) by Annibale Carracci, and A Rocky Coast, With Soldiers Studying a Plan (late 1640s) by Salvator Rosa.
A Christ Church College spokesman said staff had initially alerted police to the theft of the "important cultural artefacts", and the gallery will be closed until further notice.
Det Ch Insp Jon Capps, from Thames Valley Police, said: "The paintings which have been stolen are very high-value pieces dating back to the sixteenth and seventeenth centuries.
"The artwork has not yet been recovered but a thorough investigation is under way to find it and bring those responsible to justice."
He added there would be an increased police presence in the area, and that any witnesses or anyone with CCTV or other footage from near the area should get in touch.