We present results of a BVI variability survey in the young open cluster NGC 457 based on observations obtained during three separate runs spanning almost 20 years. In total, we found 79 variable stars, of which 66 are new. The BVI photometry was transformed to the standard system and used to derive cluster parameters by means of isochrone fitting. Using the complementary H-alpha photometry carried out in two seasons separated by over 10 years, we find that the cluster is very rich in Be stars. In total, 15 stars in the observed field of which 14 are cluster members showed H-alpha in emission either during our observations or in the past. Most of the Be stars vary in brightness on different time scales including short-period variability related most likely to g-mode pulsations. A single-epoch spectrum of NGC457-6 shows that this Be star is presently in the shell phase. The inventory of variable stars in the observed field consists of a single BCep-type star, NGC457-8, 13 Be stars, 21 slowly pulsating B stars, seven DSct stars, one GDor star, 16 unclassified periodic stars, 8 eclipsing systems and a dozen of stars with irregular variability, of which six are also B-type stars. As many as 45 variable stars are of spectral type B which is the largest number in all open clusters presented in this series of papers. The most interesting is the discovery of a large group of slowly pulsating B stars which occupy the cluster main sequence in the range between V=11 and 14.5 mag, corresponding to spectral types B3 to B8. They all have very low amplitudes and about half show pulsations with frequencies higher than 3 c/d. We argue that these are most likely fast-rotating slowly pulsating B stars, observed also in other open clusters.
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