William Lowell Putnam II (November 22, 1861 – June 1923) (more commonly known as William Putnam, Sr.) was an American lawyer and banker.
Putnam graduated from Harvard in 1882, and proceeded to make a professional name for himself in legal and financial circles. Even before his marriage to Percival Lowell's sister, Elizabeth, in 1888, Putnam (who was also Percival's half third cousin) handled a large part of the Lowell family's finances, thus leaving Augustus Lowell and Percival free to pursue their interests in science and culture. Also a successful lawyer, Putnam eventually served as the primary lawyer for both Percival Lowell and the Lowell Observatory as well as working as a partner at the well-regarded law firm of Putnam, Putnam and Bell.
Putnam's role in Lowell Observatory's history begins when Percival Lowell succumbed to severe nervous exhaustion in 1897. At this time, Putnam stepped in as the Observatory's trustee with A.E. Douglass serving as Director. Since Putnam's understanding of astronomy was minimal, he left many of the decisions about the operation of the Observatory to Douglass and contented himself with handling the Observatory's finances. As temporary trustee, he felt that it was not his place to make any dramatic changes in the Observatory's staff or location. Thus, an Observatory expedition and possibly a permanent move to South America was postponed, effectively establishing the Observatory in Flagstaff, Arizona.
Despite his conviction that he should not make any major changes to the Observatory, Putnam did become embroiled in the controversy surrounding T. J. J. See. See was employed at Lowell Observatory at that time, but was both an extremely unpopular man and also regarded as a bad scientist. Three Observatory employees had already resigned due to See's poor treatment of them, and Douglass began working to remove See from the Observatory staff. Putnam, however, had to make the final decision on the matter and he was the one to tell See that he had been dismissed.
When Lowell returned to work in 1901, Putnam resumed his role as the financial and legal power behind the Observatory. He also continued his other business in Boston and New York City.