The Sand Martin Riparia riparia is a widely distributed species, but its natural history at high elevation is poorly understood. In this paper, we describe their nesting ecology and their cooperative breeding behavior at an altitude of 3,400 m on the northeast Tibetan plateau. After arriving on their breeding grounds in early May, pairs nested solitarily, excavating nests burrows themselves into vertical stream banks. Burrows averaged 113 cm long. The clutch, averaging 4.9 pure white eggs, was laid during late May and June. Incubation, shared by both parents, lasted 14.7 days, and the young fledged at an average of 23.3 days of age. They were single-brooded and had an overall nest success of 89.5%. Cooperative breeding was observed in two out of 19 nests; in each case a single male attendant assisted the parents. This special social system has not previously been described in this species. More feeding visits were made to the cooperative nests than to non-cooperative nests and the brood size at fledging was larger in cooperative nests. In comparison with lowland populations studied in North America, Tibetan Sand Martins began nesting later, dug longer nest tunnels, produced similar clutch sizes but larger eggs, had a longer nestling period, and achieved higher nest success, suggesting an adaptive pattern to compensate for the stressful conditions at high-elevation.