Aims:We obtained the first long, homogenous time-series of V2104 Cyg, consisting of 679 datapoints, with the uvbyβ photometers of the Sierra Nevada and San Pedro Mártir Observatories. Our aim was to detect and subsequently interpret the intrinsic frequencies of this previously unstudied variable star, which turned out to be a Be star. We evaluate its place among the variable B stars on the upper Main Sequence. To obtain additional information on physical parameters we collected a few spectra with the Elodie and FIES instruments. Methods: We searched for frequencies in the uvby passbands using 2 different frequency analysis methods and used the S/N > 4 criterion to select the significant periodicities. We obtained an estimate of the physical parameters of the underlying B star of spectral type between B5 and B7, by correcting for the presence of a circumstellar disk using a formalism based on the strength of the Hα line emission. Results: We detected 3 independent frequencies with amplitudes below 0.01 mag, ν1 = 4.7126 d-1 , ν2 = 2.2342 d-1 and ν3 = 4.671 d-1 , and discovered that V2104 Cyg is a Be star. The fast rotation (vsin i = 290±10 km s-1, and 27°< i < 45°) hampered the investigation of the associated pulsational parameters ell. Conclusions: The most plausible explanation for the observed variability of this mid-late type Be star is a non-radial pulsation model. Based on observations obtained at the Observatorio Astronómico Nacional San Pedro Mártir (Mexico), Observatorio de Sierra Nevada (Spain) and Observatoire de Haute Provence (France), and on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Full Fig. 1 is only available in electronic form at http://www.aanda.org