We present the ROSAT High-Resolution Imager data of the first resolved and detected classical nova shell in the X-ray wavelengths: the shell of Nova Persei 1901. We find that the X-ray nebula is composed of knots/clumps and has an elliptical shape with a total count rate of about 0.01 +/- 0.001 counts s(-1). We estimate that the spectrum is of thermal origin with a luminosity of similar to 8.0 x 10(31) ergs s(-1) and an X-ray temperature of similar to 2.0 x 10(6) K in the 0.1-2.4 keV energy range. The knots/clumps are a result of fragmentation and condensation in the postshock region. The estimated electron density in the knots/clumps is about 10.0 cm(-3) less than or equal to n(e) less than or equal to 70.0 cm(-3). We suggest that the detected X-ray nebula could also be the reverse shock zone. This detection sheds light into one of the most poorly understood stages of the classical nova evolution.