The verb 'to surprise' is one of those verbs ending in the sound 'EYES' (IPA: /aɪz/) for which, despite the advice in -ise - -ize, the spelling with -ise "is compulsory" (Burchfield's Fowler, s.v. -ise). This spelling is also compulsory for the noun ('a surprise'). It follows the rule that when such verbs are derived from a French original, the spelling remains -ise-. The source of surprise is surpris[e], the past participle of surprendre, 'to surprise', 'to overtake with [an emotion]'.
Between the 16th and 19th centuries, both noun and verb could be spelled surprize, as can be seen in editions in original spelling of such writers as Dryden, Jane Austen and Daniel Defoe. In Robinson Crusoe (1719), the castaway on the desert island says "I was exceedingly surpriz'd with the Print of a Man's naked Foot on the Shore" as the first discovery of Man Friday.
- Etymological note: The French surprendre/surprise is constructed in the same way as the English 'overtake', which in one sense (OED's sense 5 had a similar meaning: "To overpower the will, senses, or feelings of; to win over, captivate; to overwhelm with emotion". This is most often seen in the passive, with such constructions as "the crowd was overtaken with panic", "she was overtaken by guilt" and "he was overtaken by her beauty." The earliest use of the noun surprise in English was military: 'a surprise attack' is one which the enemy is not expecting. Now, it is chiefly "The feeling or mental state, akin to astonishment and wonder, caused by an unexpected occurrence or circumstance" (OED s.v. surprise n).