Bolstered by a robust U.S. economy and newly wealthy overseas buyers, Christie’s International sold $4 billion of art during the first half of 2018, up 35% from the year-earlier period and representing a new six-month high mark for the London-based auction company.

Christie’s said on Monday that it auctioned $3.6 billion in art, up 28% from the first half of last year. The house additionally sold $390.3 million of art in privately brokered deals, more than double its year-ago results. Its online-only art sales rose 50% to...