Leonida Tonelli (19 April 1885 – 12 March 1946) was an Italian mathematician, noted for proving Tonelli's theorem, a variation of Fubini's theorem, and for introducing semicontinuity methods as a common tool for the direct method in the calculus of variations.

[1] Tonelli graduated from the University of Bologna in 1907; his Ph.D. thesis was written under the direction of Cesare Arzelà.

[2] He is one of the founders of Modern Theory of Functions of Real Variables and his work on the Calculus of Variations is a milestone in analysis.

[3]The present writer's father, W. H. Young, used to recall that this very question — what principle can we use as the foundation of the calculus of variations[4] — had been put him by a young Italian mathematician.

In the hands of Tonelli, it became an important tool in a fundamental new approach to the calculus of variations.