Using data from the family and genealogical reconstitutions of the Gitano population of 22 contiguous localities in eastern Andalusia, Spain, this study analysed the intensity, structure and historical evolution of consanguinity in 3056 couples formed from 1925 to 2006. Of these unions, 54.8% were consanguineous, and 28.7% involved relatives up to and including second cousins, resulting in a mean coefficient of inbreeding up to the third degree α3 = 12.4 × 10−3. The rest of the consanguineous unions (26.1% of all) involved more-distant relatives, such as third cousins, fourth cousins and so forth. When all consanguinity degrees found in the genealogical reconstitution were considered, the total mean coefficient of inbreeding was αt = 14.8 × 10−3. The merging of families and pedigrees generated a complex genealogical network with many inbreeding loops and important founder effects. This network revealed a high rate (62%) of Multiple Consanguineous Marriages (MCMs) in which second and subsequent consanguineous ties increased inbreeding levels by a fifth (20.5%). The accumulation of multiple degrees of distant relatedness, many of which had little social or biological importance, has contributed to a significant increase in inbreeding rates. Among Gitano people, intra-family marriages have remained common in the last decades, in sharp contrast to other Spanish populations. Hence the highest rates of close consanguinity (34%) and inbreeding (α3 = 14.6 × 10−3) were found in the 1960s, the decade that saw the onset of Spain’s socioeconomic modernization, internationalization and massive migration. These are among the highest rates of inbreeding found in any European population, including the most endogamous Spanish isolates. They reveal marriage strategies not constrained primarily by geographical barriers, but by ethnocultural separation. Interestingly, in recent decades mixed marriages have been increasing rapidly in this minority, although they are compatible with high rates of consanguinity. Gitano secular endogamy is breaking up, but not uniformly.