We present new B, V and I CCD time-series photometry for 177 variable stars in a 13 × 13arcmin2 field centred on the globular cluster M54 (lying at the centre of the Sagittarius dwarf spheroidal galaxy), 94 of which are newly identified variables. The total sample is composed of two anomalous Cepheids, 144 RR Lyrae stars (108 RR0 and 36 RR1), three SX Phoenicis, seven eclipsing binaries (five W UMa and two Algol binaries), three variables of uncertain classification and 18 long-period variables. The large majority of the RR Lyrae variables likely belong to M54. Ephemerides are provided for all the observed short-period variables. The pulsational properties of the M54 RR Lyrae variables are close to those of Oosterhoff I clusters, but a significant number of long-period ab type RR Lyrae are present. We use the observed properties of RR Lyrae to estimate the reddening and the distance modulus of M54, E(B - V) = 0.16 +/- 0.02 and (m - M)0 = 17.13 +/- 0.11, respectively, in excellent agreement with the most recent estimates. The metallicity has been estimated for a subset of 47 RR Lyrae stars, with especially good quality light curves, from the Fourier parameters of the V light curve. The derived metallicity distribution has a symmetric bell shape, with a mean of < [Fe/H]> = - 1.65 and a standard deviation σ = 0.16 dex. Seven stars have been identified as likely belonging to the Sagittarius galaxy, based on their too high or too low metallicity. This evidence, if confirmed, might suggest that old stars in this galaxy span a wide range of metallicities. Based on observations taken at the European Southern Observatory-Danish 1.54-m telescope in La Silla, Chile E-mail: asollima [at] iac.es (asollima[at]iac[dot]es)