Son-in-Law (22 April 1911 – 15 May 1941) was a British Thoroughbred racehorse and an influential sire, especially for sport horses.
The National Horseracing Museum says Son-in-Law is "probably the best and most distinguished stayer this country has ever known."
Described as "one of the principal influences for stamina in the modern thoroughbred" in Ulbrich's Peerage of Racehorses, Son-in-Law is seen in the pedigree of many of the top 100 show jumpers, particularly those of Holstein heritage.
In 1924 and 1930, Son-in-Law was the leading sire in Great Britain and Ireland. His progeny include: Foxlaw (1922-1935) - won 1927 Ascot Gold Cup Straitlace (f. 1921) - won 1924 Epsom Oaks, Coronation Stakes Rustom Pasha - extremely important sire in Argentina Suzerain (f. 1933) - third in 1937 Jockey Club Cup, Doncaster Cup Trimdon (f. 1926) - won 1931-32 Ascot Gold Cup