In 1764, Watt married his cousin Margaret (Peggy) Miller, with whom he had five children, two of whom lived to adulthood: James Jr. (1769–1848) and Margaret (1767–1796).
Në 1764, Watt u martua me kushëririn e tij Margaret (Peggy) Miller, me të cilin kishte pesë fëmijë, dy prej të cilëve jetonin në moshën madhore: James Jr (1769-1848) dhe Margaret (1767-1796).