Lazare Kaplan Sr. (1883-1986) was a Belgian-born American diamond cutter and the founder of the New York firm Lazare Kaplan International, the first house in the United States to commercially produce diamonds cut to the proportions described by Marcel Tolkowsky in his 1919 dissertation Diamond Design. Kaplan's career bridges the heroic age of the Antwerp cutting trade and the rise of New York as the world's largest market for polished diamonds, and his work established the technical and commercial template for what is still marketed today as the "ideal" or "American ideal" round brilliant.

Kaplan was born in Russia and emigrated to Antwerp as a young man, where he was apprenticed in the cutting trade and trained as a cleaver under his cousin, the cleaver and theorist Marcel Tolkowsky. After serving in the Belgian army during the First World War he relocated to New York City in 1919, where he opened a cutting workshop and began producing diamonds to Tolkowsky's calculated proportions, in particular the 40.75 degree pavilion angle, the 34.5 degree crown angle, and the 53 percent table that Tolkowsky had identified as the optimum balance of brilliance and dispersion. The brand name "Lazare Diamonds" was registered to identify these stones in the market, and the firm was for several decades the only large American manufacturer making them in quantity.

His most widely-cited technical achievement came in 1936, when he was asked by the United States government to recover a 726-carat piece of rough known as the Jonker, which had been purchased by Harry Winston earlier that year. Kaplan cleaved the rough and cut it into twelve major stones, the largest of which was the 125.65 carat Jonker emerald-cut diamond, in an operation that took nearly a year and was widely covered in the contemporary trade press as a feat of cleaving and planning. The Jonker work cemented his reputation as the most skilled cleaver in the United States and is still cited in cutting curricula.

Kaplan was also a teacher and a public advocate for cut as the principal driver of diamond beauty, an unfashionable position in the early twentieth century when colour and clarity dominated grading vocabulary. The firm he founded continues today as Lazare Kaplan International (NYSE listed for many decades), and it has remained closely associated with proportion-driven cutting and with traceability programmes such as Lazare Kaplan's "laser inscription" of every stone with a unique serial number, which the firm pioneered as a routine practice in the late 1980s.

His legacy in the trade is twofold. Technically, he proved that scientific cutting to defined proportions could be a commercial proposition rather than a one-off exhibition piece, and he created the supply chain that allowed it. Culturally, he was among the first American cutters to argue that the consumer should pay for cut as well as for the four Cs as conventionally understood, and the modern emphasis on cut grading at every major laboratory is a direct descendant of that argument. He died in New York in 1986 at the age of 102.