The Wagner Free Institute of Science is a natural history museum at 1700 West Montgomery Avenue in north Philadelphia, Pennsylvania, United States.

Its buildings, developed between 1859 and 1901, present the collections of founder William Wagner in the style of the period, and have been designated a National Historic Landmark for their architecture and state of preservation.

Wagner began offering free lectures on science at his home, Elm Grove, a colonial farm estate on the outskirts of Philadelphia in 1847.

Leidy personally developed and supervised their reorganization into a systematic display in which specimens and cases were arranged according to Darwin's theory of evolution, so that visitors moved from simpler to more complex organisms and through geologic time as they walked through the exhibition hall.

[6] The Institute has continued to carry out Wagner's initial goals of offering free science courses, while also functioning as a library, lecture hall and museum.