Lucas Arnold Ker (Spanish pronunciation: [ˈlukas aɾˈnol ˈkeɾ];[1] born 12 October 1974) is an Argentine former professional tennis player.

Arnold Ker started playing tennis in the Olivos Tenis Club of Buenos Aires, under coach Malcolm Campbell.

He played several times for the Argentine Davis Cup team, where he made his debut in 1997 against Venezuela.

Arnold Ker was Roger Federer's opponent in the latter's first professional match in 1998, beating the 17-year-old future singles world No.

Arnold Ker retired from professional tennis in 2016.