Seasonal changes in the species composition and fibre content of plant fragments in the rumen and the gross anatomy of the gastrointestinal tract were investigated in Svalbard reindeer Rangifer tarandus platyrhynchus in western parts of Spitsbergen (Nordenskiöld Land (NL), 78°N), where the tundra vegetation is relatively abundant, and on the island of Nordaustlandet ((NA) 81°N) where, in the polar desert, plants are scarce. Reindeer were killed while grazing on NL in autumn (n = 19) and in winter (n = 10) and on NA in summer (n = 10) and in winter (n = 3). Rumen contents included mainly mosses and grasses in specimens from NL and mainly Saxifraga spp. in specimens from NA. The plant cell wall content of the dry organic matter in the rumen-reticulum (RR) ranged from 55% to 77% and from 43% to 57% in NL and NA specimens, respectively, and did not vary between seasons. In autumn, the wet weight of the contents of the RR ranged from 13% to 20% of body mass (BM) in females on NL and from 14% to 24% of BM in adult reindeer in winter. On NA, the wet weight of the contents of the RR in adult animals ranged from 15% to 20% of BM in summer and from 15% to 17% of BM in winter in NA. The ratio of the wet weight of the contents of the distal fermentation chamber (DFC) to the wet weight of the contents of the RR was 1:8 in females in autumn, compared to 1:6–1:7 in adult animals on NL in winter. The ratio of the wet weight of the DFC to the wet weight of the RR ranged between 1:8–1:10 in summer, compared to 1:7 and 1:8 in winter. There were no significant differences in the size of plant particles in the RR contents in females from the two areas in any season, indicating that ruminal plant particle size was not influenced by the concentration of plant cell walls in the rumen contents. We conclude that the degree of filling of the gastrointestinal tract of these reindeer was therefore principally a function of plant fibre chemistry. Svalbard reindeer are adaptable intermediate feeders but they have, nevertheless, developed a large DFC similar to that of concentrate selectors.