Loyal Edwin Knappen (January 27, 1854 – May 14, 1930) was a United States federal judge.
Born in Hastings, Michigan, Knappen received a B.A. from the University of Michigan in 1873 and read law to enter the bar in 1875, thereafter receiving an M.A. from the University of Michigan in 1876. He was in private practice in Hastings, Michigan from 1875 to 1888, also serving as a prosecuting attorney of Barry County, Michigan from 1879 to 1883. He was a Commissioner for the U.S. District Court for the Western District of Michigan from 1880 to 1888. He was in private practice in Grand Rapids, Michigan from 1888 to 1906 with the law firm of Knappen, Uhl & Bryant, which had been founded as Fletcher & Wanty, and which continues to exist today as Wheeler Upham.
On December 3, 1906, Knappen was nominated by President Theodore Roosevelt to a seat on the United States District Court for the Western District of Michigan vacated by the death of George P. Wanty, who had founded the law firm of which Knappen was a partner. Knappen was confirmed by the United States Senate on December 10, 1906, and received his commission the same day.
On January 17, 1910, President William H. Taft nominated Knappen for elevation to a seat on the United States Court of Appeals for the Sixth Circuit vacated by the successful nomination of Horace Harmon Lurton to the Supreme Court of the United States. Knappen was confirmed by the United States Senate on January 31, 1910, and received his commission the same day. He assumed senior status on April 15, 1924, serving in that capacity until his death, in 1930.