Aims: We are performing a search for symbiotic stars using IPHAS, the INT Hα survey of the northern Galactic plane, and follow-up observations. Methods: Candidate symbiotic stars are selected on the basis of their IPHAS and near-IR colours, and spectroscopy and photometry are obtained to determine their nature. We present here observations of the symbiotic star candidate IPHAS J205836.43+503307.2. Results: The optical spectrum shows the combination of a number of emission lines, among which are the high-excitation species of [O iii], He ii, [Ca v], and [Fe vii], and a red continuum with the features of a star at the cool end of the carbon star sequence. The nebular component is spatially resolved: the analysis of the spatial profile of the [N ii]6583 line in the spectrum indicates a linear size of ~ 2.5 arcsec along the east-west direction. Its velocity structure suggests an aspherical morphology. The near-infrared excess of the source, which was especially strong in 1999, indicated that a thick circumstellar dust shell was also present in the system. The carbon star has brightened in the last decade by two to four magnitudes at red and near-infrared wavelengths. Photometric monitoring during a period of 60 days from November 2010 to January 2011 reveals a slow luminosity decrease of 0.2 mag. Conclusions: From the observed spectrophotometric properties and variability, we conclude that the source is a new Galactic symbiotic star of the D-type, of the rare kind that contains a carbon star, likely a carbon Mira. Only two other systems of this type are known in the Galaxy. Based on observations obtained with the 2.5 m INT and the 4.2 m WHT telescopes of the Isaac Newton Group of Telescopes and the 1.5 m Carlos Sanchez Telescope, operating on the islands of La Palma and Tenerife at the Spanish Observatories of the Roque de Los Muchachos and Teide of the Instituto de Astrofísica de Canarias; the 2.1 m telescope at San Pedro Martir, Mexico; and the GAPC 0.7 m Ritchey-Chrétien telescope at La Polse di Cougnes, Udine, Italy.