It drew at least four interested parties before it was knocked down to London-based dealer Jean-Luc Baroni at £11m.
It was purchased for a client.
Surpassing its £3m-5m estimate, the price was the highest ever paid for a French Old Master at auction. The previous record for Watteau was set back in December 2000 when Christie's sold Le conteur, also in London, for a premium-inclusive £2,423,750.
Speaking to ATG after the sale, Mr Baroni said that La Surprise was an exceptional work, not only the best ever Watteau to appear on the market but also incredibly well preserved.
The 14 1/2 x 11 1/2in (36 x 28cm) oil on panel from c.1718 was previously only known thanks to a copy in the Royal Collection at Buckingham Palace and through a contemporary engraving. It was found in the corner of a drawing room in a British country house during a Christie's valuation last year.
The subject matter - lovers in a parkland paradise accompanied by the character Mezzetin from the Comedie Italienne tuning his guitar, and a small dog - was deemed particularly commercial.
Its provenance also added to its appeal. It was first owned by Nicolas Henin (1691-1724), an advisor to the French King. It is likely that the work was painted for Henin together with its companion piece L'Accord Parfait, now in the Los Angeles Museum of Art.
The Christie's sale also saw a record for Anthony van Dyke (1599-1641), when A Rearing Stallion sold to a US collector on the telephone at £2.7m.
Although the hammer total for the evening sale was £21m, not all of the 48 lots witnessed such levels of demand. Seventeen pictures went unsold.
Earlier in the day, Christie's had sold three rediscovered drawings by Francisco José de Goya y Lucientes (1746-1828) for a combined total of £3,470,000, all to different European private buyers.
Christie's had the top-priced picture in the Old Master series but Sotheby's comfortably the better evening sale on July 9, boasting a higher number of commercial Dutch and Flemish works to appeal to the Old Master trade.
Richard Green, Konrad Bernheimer, Johnny van Haeften and Jean-Luc Baroni were among the buyers of the 13 works sold for more than £1m. The hammer total was £44.6m with 69 of the 90 lots sold.
The outstanding Dutch picture in the sale was Frans Hals' (1580-1566) portrait of the Harlam textiles merchant Willem van Heythuysen estimated at £3-5m. This 18 1/2in (47cm) panel had been sold at auction in Vienna in 2004 as the work of a 'studio or follower' for a premium-inclusive €571,120 (£416,875).
Subsequent research, including analysis of the single plank oak panel, has suggested this is the primary version of a familiar image dating from the mid-1630s. London agent Richard Nagy was the buyer in the room at £6.3m.
The final lot of the sale was J.M.W. Turner (1775-1851) Pope's Villa at Twickenham of 1808, sold to benefit Sudeley Castle in Gloucestershire and expected to bring £5-7m.
Guaranteeing this early Claudian work that lacked the presence of the artist's highly-rated later paintings looked quite a gamble for Sotheby's but a lone telephone bid from a private collector saw it sell at £4.8m.