In this paper, we present the results obtained in the determination of the true North direction on Mars by using a gnomon on the InSight mission and compare the measurements with either the North determination from the Inertial Measurement Unit and Imaging analysis. The obtained measurement has been used to populate the SEIS orientation information in the archived SEIS data. Images taken during December 2018 and January 2019 allow to determine the gnomon shadow position and length over a target. By calculating the Sun local coordinates using planetary ephemeris VSOP87, the images are used to estimate the true North direction on the landing site. By using eight different images selected, we obtain the true North direction with an accuracy up to 2.5 • , which is confirmed by the IMU and Imaging analysis. The true North direction is also confirmed by an image taken near local noon, when the sun crosses the meridian. The North determination precision is then discussed in view of the seismic determination of the back-azimuth. NASA's InSight landing on Mars took place on November 26, 2018. After a few weeks of surface operations, InSight deployed the seismometer package SEIS (Seismic Experience for Interior Structure) (Lognonné et al., 2019) on the martian surface and provided first results (Banerdt et al., 2020; Giardini et al., 2020; Lognonné et al., 2020). Many of the SEIS scientific investigations were made by the 3 axis capability of the SEIS instrument, enabling the measurement, in the North, East, vertical local reference, of the ground acceleration. This paper is therefore focused on the methods used to determine this reference frame. While SEIS has its vertical axis, thanks to its leveling system, aligned with the local gravity to within less than 0.1 • , the determination of its