The physical and chemical characteristics of chyme have been shown to relate to apparent faecal nutrient digestibility, also in fish. This study assessed the effect of dietary viscosity on chyme characteristics in different segments of the gastrointestinal tract (GIT) of striped catfish (Pangasianodon hypophthalmus, Sauvage, 1878). Moreover, its effect on the progression of nutrient digestion throughout the GIT was studied. Six diets were produced, that had moderate differences in dietary viscosity by exchanging carboxymethylcellulose for guar gum (0, 0.25, 0.5, 1, 2, or 3 g kg−1 of diet). Eighteen tanks, each stocked with 20 fish of 95 g, were used (three replicate tanks per diet). Diets were randomly assigned to tanks and fed for 29 days. Thereafter fish were euthanized for the collection of chyme from four different segments of the GIT: stomach, proximal, mid and distal intestine. The chyme characteristics, dry matter content, viscosity and osmolarity were affected by the interaction effect of diet and GIT-segment. This implies that the impact of dietary viscosity on chyme characteristics differs between GIT-segments. Chyme viscosity increased with increasing dietary viscosity, but this mainly occurred in the stomach and distal intestine. Dry matter content of chyme was mainly affected (increased with rising dietary viscosity) in the stomach. The digestibility of both dry matter and crude protein was different between GIT-segments and increased from stomach towards distal intestine. Moreover, these nutrient digestibilities were influenced by dietary viscosity being negatively related. Already in the stomach increased dietary viscosity reduced the disappearance of nutrients (i.e., digestibility). These differences in digestibility were persistent and constant throughout the total GIT. In conclusion, moderate increases in dietary viscosity: alter predominantly stomach and distal chyme viscosity; increases chyme DM content in distal intestine; and reduces protein and DM digestibility, which are already different between diets from the stomach onward.