We present a catalogue of X-ray selected galaxy clusters and groups as a first release of the 2XMMi/SDSS Galaxy Cluster Survey. The survey is a search for galaxy clusters detected serendipitously in observations with XMM-Newton in the footprint of the Sloan Digital Sky Survey (SDSS). The main aims of the survey are to identify new X-ray galaxy clusters, investigate their X-ray scaling relations, identify distant cluster candidates, and study the correlation of the X-ray and optical properties. In this paper, we describe the basic strategy to identify and characterize the X-ray cluster candidates that currently comprise 1180�objects selected from the second XMM-Newton serendipitous source catalogue (2XMMi-DR3). Cross-correlation of the initial catalogue with recently published optically selected SDSS galaxy cluster catalogues yields photometric redshifts for 275�objects. Of these, 182�clusters have at least one member with a spectroscopic redshift from existing public data (SDSS-DR8). We developed an automated method to reprocess the XMM-Newton X-ray observations, determine the optimum source extraction radius, generate source and background spectra, and derive the temperatures and luminosities of the optically confirmed clusters. Here we present the X-ray properties of the first cluster sample, which comprises 175 clusters, among which 139�objects are new X-ray discoveries while the others were previously known as X-ray sources. For each cluster, the catalogue provides: two identifiers, coordinates, temperature, flux [0.5−2]�keV, luminosity [0.5−2]�keV extracted from an optimum aperture, bolometric luminosity L₅₀₀, total mass M₅₀₀, radius R₅₀₀, and the optical properties of the counterpart. The first cluster sample from the survey covers a wide range of redshifts from 0.09 to 0.61, bolometric luminosities L₅₀₀�=�1.9�נ10⁴²−1.2�נ10⁴⁵ erg s^-1, and masses M₅₀₀�=�2.3�נ10¹³−4.9�נ10¹⁴ M_⊙. We extend the relation between the X-ray bolometric luminosity L₅₀₀ and the X-ray temperature towards significantly lower T and L and still find that the slope of the linear L�−�T relation is consistent with values published for high luminosities.