Phi Mu (ΦΜ) is the second oldest female fraternal organization established in the United States.

The fraternity was founded at Wesleyan College in Macon, Georgia as the Philomathean Society on January 4, 1852, and was announced publicly on March 4 of the same year. Phi Mu is one of the two "Macon Magnolias," a term used to celebrate the bonds it shares with Alpha Delta Pi as sororities founded on that same campus.

Today, Phi Mu has 136 active collegiate chapters, 137 active alumnae chapters, and more than 187,000 initiated sisters. In its 170-year history, Phi Mu has chartered over 228 chapters. Phi Mu's National Headquarters is in Peachtree City, Georgia. Phi Mu's national philanthropy is Children's Miracle Network Hospitals.

Phi Mu is one of 26 national sororities which are members under the umbrella organization of the National Panhellenic Conference.

Phi Mu was founded on January 4, 1852 – though not publicly announced until March 4, 1852 – originally as a literary society referred to as The Philomathean Society at Wesleyan College by Mary Ann Dupont (Lines), Mary Elizabeth Myrick (Daniel), and Martha Bibb Hardaway (Redding). Philomathean is derived from the Greek philomath, which means a lover of learning. In 1904, the society was established as a national organization by the State of Georgia taking on the Greek letters Phi Mu. Phi Mu joined the National Panhellenic Conference in 1911.

Phi Mu is referred to as a women's fraternity since some sororities predate the term "sorority" and are thus known as "fraternities for women." Phi Mu is one such sorority, and its formal name is Phi Mu Fraternity.

In 1939, Alpha Delta Theta, a small national sorority founded at Transylvania University, merged with Phi Mu.

Although Phi Mu has no official jewel, the fraternity recognizes the official colors of rose, symbolizing womanhood, and white, symbolizing truth and purity. Its flower is the rose-colored carnation. Its symbol is a quatrefoil. Phi Mu's official mascot is a lion named Sir Fidel.