  We report on the discovery of a transiting Earth-sized (0.95$R_\oplus$)
planet around an M3.5 dwarf star at 57$\,$pc, K2-315b. The planet has a period
of $\sim$3.14 days, i.e. ${\sim}\pi$, with an instellation of
7.45$\,$S$_{\oplus}$. The detection was made using publicly available data from
$\textit{K2}$'s Campaign 15. We observed three additional transits with
SPECULOOS Southern and Northern Observatories, and a stellar spectrum from
Keck/HIRES, which allowed us to validate the planetary nature of the signal.
The confirmed planet is well suited for comparative terrestrial exoplanetology.
While exoplanets transiting ultracool dwarfs present the best opportunity for
atmospheric studies of terrestrial exoplanets with the $\textit{James Webb
Space Telescope}$, those orbiting mid-M dwarfs within 100$\,$pc such as K2-315b
will become increasingly accessible with the next generation of observatories.
