In this paper we directly constrain possible spatial variations of the Newtonian gravitational constant G over ranges \approx 0.01 - 5 AU in various extrasolar multi-planet systems .
By means of the third Kepler ’ s law we determine the quantity \Gamma _ { XY } = G _ { X } / G _ { Y } for each couple of planets X and Y located at different distances from their parent star : deviations of the measured values of \Gamma from unity would signal variations of G .
The obtained results for \eta = 1 - \Gamma are found to be well compatible with zero within the experimental errors ( \eta / \delta \eta \approx 0.2 - 0.3 ) .
A comparison with an analogous test previously performed in our Solar System is made .