HAT-P-38b, formally named Hiisi, is a transiting exoplanet orbiting the G-type star HAT-P-38 821 light-years from the Solar System in the constellation Triangulum.

The approved name for the planet is Hiisi, which represents sacred localities and later evil spirits from Finnic mythology.

[1] Later observations of the star's radial velocity using the High Dispersion Spectrograph confirmed HAT-P-38b's planetary status and it also yielded its mass in the process.

[1] HAT-P-38b is a hot Saturn that takes over 4 days to circle its host star in a relatively circular and tight orbit; it has a separation of 0.0519 astronomical units.

[5] Giovanni Bruno and colleagues detected water vapor in both planets, with HAT-P-38b having the greater abundance in its atmosphere.